mm, memcg: cg2 memory{.swap,}.peak write handlers

mm, memcg: cg2 memory{.swap,}.peak write handlers

[David Finkel]

This is an updated patchset following some excellent comments from Roman
and Longman. [1]

As suggested, I've broken this into two commits:
 1) with the implementation changes
 2) extending the tests tests

I haven't been able to induce anything problematic, but I'm a bit
unclear as to whether there's reference counting on cgroups such that
we don't need to handle the case where the cgroup is freed before the
one of the peak files is closed.


 Documentation/admin-guide/cgroup-v2.rst          |  26 ++-
 include/linux/cgroup.h                           |   8 +
 include/linux/memcontrol.h                       |   5 +
 include/linux/page_counter.h                     |  11 +-
 kernel/cgroup/cgroup-internal.h                  |   2 +
 kernel/cgroup/cgroup.c                           |   7 +
 mm/memcontrol.c                                  | 129 +++++++++++++--
 mm/page_counter.c                                |  36 ++++-
 tools/testing/selftests/cgroup/cgroup_util.c     |  22 +++
 tools/testing/selftests/cgroup/cgroup_util.h     |   2 +
 tools/testing/selftests/cgroup/test_memcontrol.c | 227 ++++++++++++++++++++++++++-
 11 files changed, 444 insertions(+), 31 deletions(-)


[1]: https://lore.kernel.org/cgroups/20240722151713.2724855-1-davidf@vimeo.com/T/


Thank you for your efforts and reviews,

David Finkel
Senior Principal Software Engineer
Vimeo Inc.

[PATCH 1/2] mm, memcg: cg2 memory{.swap,}.peak write handlers

[David Finkel]

Other mechanisms for querying the peak memory usage of either a process
or v1 memory cgroup allow for resetting the high watermark. Restore
parity with those mechanisms, but with a less racy API.

For example:
 - Any write to memory.max_usage_in_bytes in a cgroup v1 mount resets
   the high watermark.
 - writing "5" to the clear_refs pseudo-file in a processes's proc
   directory resets the peak RSS.

This change is an evolution of a previous patch, which mostly copied the
cgroup v1 behavior, however, there were concerns about races/ownership
issues with a global reset, so instead this change makes the reset
filedescriptor-local.

Writing a specific string to the memory.peak and memory.swap.peak
pseudo-files reset the high watermark to the current usage for
subsequent reads through that same fd.

Notably, following Johannes's suggestion, this implementation moves the
O(fds that have written) behavior onto the fd write(2) path. Instead, on
the page-allocation path, we simply add one additional watermark to
conditionally bump per-hierarchy level in the page-counter.

Additionally, this takes Longman's suggestion of nesting the
page-charging-path checks for the two watermarks to reduce the number of
common-case comparisons.

This behavior is particularly useful for work scheduling systems that
need to track memory usage of worker processes/cgroups per-work-item.
Since memory can't be squeezed like CPU can (the OOM-killer has
opinions), these systems need to track the peak memory usage to compute
system/container fullness when binpacking workitems.

Most notably, Vimeo's use-case involves a system that's doing global
binpacking across many Kubernetes pods/containers, and while we can use
PSI for some local decisions about overload, we strive to avoid packing
workloads too tightly in the first place. To facilitate this, we track
the peak memory usage. However, since we run with long-lived workers (to
amortize startup costs) we need a way to track the high watermark while
a work-item is executing. Polling runs the risk of missing short spikes
that last for timescales below the polling interval, and peak memory
tracking at the cgroup level is otherwise perfect for this use-case.

As this data is used to ensure that binpacked work ends up with
sufficient headroom, this use-case mostly avoids the inaccuracies
surrounding reclaimable memory.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: David Finkel <davidf@vimeo.com>
---
 Documentation/admin-guide/cgroup-v2.rst      |  26 ++--
 include/linux/cgroup.h                       |   8 ++
 include/linux/memcontrol.h                   |   5 +
 include/linux/page_counter.h                 |  11 +-
 kernel/cgroup/cgroup-internal.h              |   2 +
 kernel/cgroup/cgroup.c                       |   7 +
 mm/memcontrol.c                              | 129 +++++++++++++++++--
 mm/page_counter.c                            |  36 +++++-
 tools/testing/selftests/cgroup/cgroup_util.c |  22 ++++
 tools/testing/selftests/cgroup/cgroup_util.h |   2 +
 10 files changed, 225 insertions(+), 23 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 8fbb0519d556..8dd68e07d13d 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1322,11 +1322,16 @@ PAGE_SIZE multiple when read back.
 	reclaim induced by memory.reclaim.
 
   memory.peak
-	A read-only single value file which exists on non-root
-	cgroups.
+	A read-write single value file which exists on non-root cgroups.
+
+	The max memory usage recorded for the cgroup and its descendants since
+	either the creation of the cgroup or the most recent reset for that FD.
 
-	The max memory usage recorded for the cgroup and its
-	descendants since the creation of the cgroup.
+	A write of the string "reset" to this file resets it to the
+	current memory usage for subsequent reads through the same
+	file descriptor.
+	Attempts to write any other non-empty string will return EINVAL
+	(modulo leading and trailing whitespace).
 
   memory.oom.group
 	A read-write single value file which exists on non-root
@@ -1652,11 +1657,16 @@ PAGE_SIZE multiple when read back.
 	Healthy workloads are not expected to reach this limit.
 
   memory.swap.peak
-	A read-only single value file which exists on non-root
-	cgroups.
+	A read-write single value file which exists on non-root cgroups.
+
+	The max swap usage recorded for the cgroup and its descendants since
+	the creation of the cgroup or the most recent reset for that FD.
 
-	The max swap usage recorded for the cgroup and its
-	descendants since the creation of the cgroup.
+	A write of the string "reset" to this file resets it to the
+	current memory usage for subsequent reads through the same
+	file descriptor.
+	Attempts to write any other non-empty string will return EINVAL
+	(modulo leading and trailing whitespace).
 
   memory.swap.max
 	A read-write single value file which exists on non-root
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2150ca60394b..7001ed74e339 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -12,6 +12,7 @@
 #include <linux/sched.h>
 #include <linux/cpumask.h>
 #include <linux/nodemask.h>
+#include <linux/list.h>
 #include <linux/rculist.h>
 #include <linux/cgroupstats.h>
 #include <linux/fs.h>
@@ -855,4 +856,11 @@ static inline void cgroup_bpf_put(struct cgroup *cgrp) {}
 
 struct cgroup *task_get_cgroup1(struct task_struct *tsk, int hierarchy_id);
 
+struct memcg_peak_mem_ctx {
+	long				local_watermark;
+	struct list_head		peers;
+};
+
+struct memcg_peak_mem_ctx *memcg_extract_peak_mem_ctx(struct kernfs_open_file *of);
+
 #endif /* _LINUX_CGROUP_H */
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 030d34e9d117..cbc390234605 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -198,6 +198,11 @@ struct mem_cgroup {
 	struct page_counter kmem;		/* v1 only */
 	struct page_counter tcpmem;		/* v1 only */
 
+	/* lists of memcg peak watching contexts on swap and memory */
+	struct list_head peak_memory_local_watermark_watchers;
+	struct list_head peak_swap_local_watermark_watchers;
+	spinlock_t swap_memory_peak_watchers_lock;
+
 	/* Range enforcement for interrupt charges */
 	struct work_struct high_work;
 
diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index 8cd858d912c4..06bb84218960 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -26,6 +26,7 @@ struct page_counter {
 	atomic_long_t children_low_usage;
 
 	unsigned long watermark;
+	unsigned long local_watermark; /* track min of fd-local resets */
 	unsigned long failcnt;
 
 	/* Keep all the read most fields in a separete cacheline. */
@@ -78,7 +79,15 @@ int page_counter_memparse(const char *buf, const char *max,
 
 static inline void page_counter_reset_watermark(struct page_counter *counter)
 {
-	counter->watermark = page_counter_read(counter);
+	unsigned long cur = page_counter_read(counter);
+
+	counter->watermark = cur;
+	counter->local_watermark = cur;
+}
+
+static inline void page_counter_reset_local_watermark(struct page_counter *counter)
+{
+	counter->local_watermark = page_counter_read(counter);
 }
 
 #endif /* _LINUX_PAGE_COUNTER_H */
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 520b90dd97ec..5a97ba08e976 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -81,6 +81,8 @@ struct cgroup_file_ctx {
 	struct {
 		struct cgroup_pidlist	*pidlist;
 	} procs1;
+
+	struct memcg_peak_mem_ctx peak;
 };
 
 /*
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e32b6972c478..38b935ffa6cf 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1964,6 +1964,13 @@ static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param
 	return -EINVAL;
 }
 
+struct memcg_peak_mem_ctx *memcg_extract_peak_mem_ctx(struct kernfs_open_file *of)
+{
+	struct cgroup_file_ctx *ctx = of->priv;
+
+	return &ctx->peak;
+}
+
 static void apply_cgroup_root_flags(unsigned int root_flags)
 {
 	if (current->nsproxy->cgroup_ns == &init_cgroup_ns) {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8f2f1bb18c9c..306698d32828 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -25,6 +25,7 @@
  * Copyright (C) 2020 Alibaba, Inc, Alex Shi
  */
 
+#include <linux/cgroup-defs.h>
 #include <linux/page_counter.h>
 #include <linux/memcontrol.h>
 #include <linux/cgroup.h>
@@ -42,6 +43,7 @@
 #include <linux/rcupdate.h>
 #include <linux/limits.h>
 #include <linux/export.h>
+#include <linux/list.h>
 #include <linux/mutex.h>
 #include <linux/rbtree.h>
 #include <linux/slab.h>
@@ -5745,6 +5747,9 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
 	vmpressure_init(&memcg->vmpressure);
 	INIT_LIST_HEAD(&memcg->event_list);
 	spin_lock_init(&memcg->event_list_lock);
+	INIT_LIST_HEAD(&memcg->peak_memory_local_watermark_watchers);
+	INIT_LIST_HEAD(&memcg->peak_swap_local_watermark_watchers);
+	spin_lock_init(&memcg->swap_memory_peak_watchers_lock);
 	memcg->socket_pressure = jiffies;
 #ifdef CONFIG_MEMCG_KMEM
 	memcg->kmemcg_id = -1;
@@ -6907,12 +6912,109 @@ static u64 memory_current_read(struct cgroup_subsys_state *css,
 	return (u64)page_counter_read(&memcg->memory) * PAGE_SIZE;
 }
 
-static u64 memory_peak_read(struct cgroup_subsys_state *css,
-			    struct cftype *cft)
+inline int swap_memory_peak_show(
+	struct seq_file *sf, void *v, bool swap_cg)
 {
+	struct cgroup_subsys_state *css = seq_css(sf);
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	struct page_counter *pc;
+	struct kernfs_open_file *of = sf->private;
+	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
+	s64 fd_peak = ctx->local_watermark;
 
-	return (u64)memcg->memory.watermark * PAGE_SIZE;
+	if (swap_cg)
+		pc = &memcg->swap;
+	else
+		pc = &memcg->memory;
+
+	if (fd_peak == -1) {
+		seq_printf(sf, "%llu\n", (u64)pc->watermark * PAGE_SIZE);
+		return 0;
+	}
+
+	s64 pc_peak = pc->local_watermark;
+	s64 wm = fd_peak > pc_peak ? fd_peak : pc_peak;
+
+	seq_printf(sf, "%lld\n", wm * PAGE_SIZE);
+	return 0;
+}
+
+static int memory_peak_show(struct seq_file *sf, void *v)
+{
+	return swap_memory_peak_show(sf, v, false);
+}
+
+static int swap_memory_peak_open(struct kernfs_open_file *of)
+{
+	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
+
+	ctx->local_watermark = -1;
+	return 0;
+}
+
+inline void swap_memory_peak_release(struct kernfs_open_file *of)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
+
+	if (ctx->local_watermark == -1) {
+		/* fast path (no writes on this fd) */
+		return;
+	}
+	spin_lock(&memcg->swap_memory_peak_watchers_lock);
+	list_del(&ctx->peers);
+	spin_unlock(&memcg->swap_memory_peak_watchers_lock);
+}
+
+inline ssize_t swap_memory_peak_write(
+	struct kernfs_open_file *of,
+	char *buf, size_t nbytes, loff_t off, bool swap_cg)
+{
+	unsigned long cur;
+	struct memcg_peak_mem_ctx *peer_ctx;
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
+	struct page_counter *pc;
+	struct list_head *watchers, *pos;
+
+	buf = strstrip(buf);
+	/* Only allow "reset" to keep the API clear */
+	if (strcmp(buf, "reset"))
+		return -EINVAL;
+
+	if (swap_cg) {
+		pc = &memcg->swap;
+		watchers = &memcg->peak_swap_local_watermark_watchers;
+	} else {
+		pc = &memcg->memory;
+		watchers = &memcg->peak_memory_local_watermark_watchers;
+	}
+
+	spin_lock(&memcg->swap_memory_peak_watchers_lock);
+
+	page_counter_reset_local_watermark(pc);
+	cur = pc->local_watermark;
+
+	list_for_each(pos, watchers) {
+		peer_ctx = list_entry(pos, typeof(*ctx), peers);
+		if (cur > peer_ctx->local_watermark)
+			peer_ctx->local_watermark = cur;
+	}
+
+	if (ctx->local_watermark == -1)
+		/* only append to the list if we're not already there */
+		list_add_tail(&ctx->peers, watchers);
+
+	ctx->local_watermark = cur;
+	spin_unlock(&memcg->swap_memory_peak_watchers_lock);
+
+	return nbytes;
+}
+
+static ssize_t memory_peak_write(struct kernfs_open_file *of, char *buf,
+				 size_t nbytes, loff_t off)
+{
+	return swap_memory_peak_write(of, buf, nbytes, off, false);
 }
 
 static int memory_min_show(struct seq_file *m, void *v)
@@ -7231,7 +7333,10 @@ static struct cftype memory_files[] = {
 	{
 		.name = "peak",
 		.flags = CFTYPE_NOT_ON_ROOT,
-		.read_u64 = memory_peak_read,
+		.open = swap_memory_peak_open,
+		.release = swap_memory_peak_release,
+		.seq_show = memory_peak_show,
+		.write = memory_peak_write,
 	},
 	{
 		.name = "min",
@@ -8193,12 +8298,15 @@ static u64 swap_current_read(struct cgroup_subsys_state *css,
 	return (u64)page_counter_read(&memcg->swap) * PAGE_SIZE;
 }
 
-static u64 swap_peak_read(struct cgroup_subsys_state *css,
-			  struct cftype *cft)
+static int swap_peak_show(struct seq_file *sf, void *v)
 {
-	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	return swap_memory_peak_show(sf, v, true);
+}
 
-	return (u64)memcg->swap.watermark * PAGE_SIZE;
+static ssize_t swap_peak_write(struct kernfs_open_file *of, char *buf,
+			       size_t nbytes, loff_t off)
+{
+	return swap_memory_peak_write(of, buf, nbytes, off, true);
 }
 
 static int swap_high_show(struct seq_file *m, void *v)
@@ -8282,7 +8390,10 @@ static struct cftype swap_files[] = {
 	{
 		.name = "swap.peak",
 		.flags = CFTYPE_NOT_ON_ROOT,
-		.read_u64 = swap_peak_read,
+		.open = swap_memory_peak_open,
+		.release = swap_memory_peak_release,
+		.seq_show = swap_peak_show,
+		.write = swap_peak_write,
 	},
 	{
 		.name = "swap.events",
diff --git a/mm/page_counter.c b/mm/page_counter.c
index db20d6452b71..724d31508664 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -79,9 +79,22 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
 		/*
 		 * This is indeed racy, but we can live with some
 		 * inaccuracy in the watermark.
+		 *
+		 * Notably, we have two watermarks to allow for both a globally
+		 * visible peak and one that can be reset at a smaller scope.
+		 *
+		 * Since we reset both watermarks when the global reset occurs,
+		 * we can guarantee that watermark >= local_watermark, so we
+		 * don't need to do both comparisons every time.
+		 *
+		 * On systems with branch predictors, the inner condition should
+		 * be almost free.
 		 */
-		if (new > READ_ONCE(c->watermark))
-			WRITE_ONCE(c->watermark, new);
+		if (new > READ_ONCE(c->local_watermark)) {
+			WRITE_ONCE(c->local_watermark, new);
+			if (new > READ_ONCE(c->watermark))
+				WRITE_ONCE(c->watermark, new);
+		}
 	}
 }
 
@@ -131,10 +144,23 @@ bool page_counter_try_charge(struct page_counter *counter,
 		propagate_protected_usage(c, new);
 		/*
 		 * Just like with failcnt, we can live with some
-		 * inaccuracy in the watermark.
+		 * inaccuracy in the watermarks.
+		 *
+		 * Notably, we have two watermarks to allow for both a globally
+		 * visible peak and one that can be reset at a smaller scope.
+		 *
+		 * Since we reset both watermarks when the global reset occurs,
+		 * we can guarantee that watermark >= local_watermark, so we
+		 * don't need to do both comparisons every time.
+		 *
+		 * On systems with branch predictors, the inner condition should
+		 * be almost free.
 		 */
-		if (new > READ_ONCE(c->watermark))
-			WRITE_ONCE(c->watermark, new);
+		if (new > READ_ONCE(c->local_watermark)) {
+			WRITE_ONCE(c->local_watermark, new);
+			if (new > READ_ONCE(c->watermark))
+				WRITE_ONCE(c->watermark, new);
+		}
 	}
 	return true;
 
diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
index 432db923bced..1e2d46636a0c 100644
--- a/tools/testing/selftests/cgroup/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/cgroup_util.c
@@ -141,6 +141,16 @@ long cg_read_long(const char *cgroup, const char *control)
 	return atol(buf);
 }
 
+long cg_read_long_fd(int fd)
+{
+	char buf[128];
+
+	if (pread(fd, buf, sizeof(buf), 0) <= 0)
+		return -1;
+
+	return atol(buf);
+}
+
 long cg_read_key_long(const char *cgroup, const char *control, const char *key)
 {
 	char buf[PAGE_SIZE];
@@ -183,6 +193,18 @@ int cg_write(const char *cgroup, const char *control, char *buf)
 	return ret == len ? 0 : ret;
 }
 
+/*
+ * Returns fd on success, or -1 on failure.
+ * (fd should be closed with close() as usual)
+ */
+int cg_open(const char *cgroup, const char *control, int flags)
+{
+	char path[PATH_MAX];
+
+	snprintf(path, sizeof(path), "%s/%s", cgroup, control);
+	return open(path, flags);
+}
+
 int cg_write_numeric(const char *cgroup, const char *control, long value)
 {
 	char buf[64];
diff --git a/tools/testing/selftests/cgroup/cgroup_util.h b/tools/testing/selftests/cgroup/cgroup_util.h
index e8d04ac9e3d2..19b131ee7707 100644
--- a/tools/testing/selftests/cgroup/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/cgroup_util.h
@@ -34,9 +34,11 @@ extern int cg_read_strcmp(const char *cgroup, const char *control,
 extern int cg_read_strstr(const char *cgroup, const char *control,
 			  const char *needle);
 extern long cg_read_long(const char *cgroup, const char *control);
+extern long cg_read_long_fd(int fd);
 long cg_read_key_long(const char *cgroup, const char *control, const char *key);
 extern long cg_read_lc(const char *cgroup, const char *control);
 extern int cg_write(const char *cgroup, const char *control, char *buf);
+extern int cg_open(const char *cgroup, const char *control, int flags);
 int cg_write_numeric(const char *cgroup, const char *control, long value);
 extern int cg_run(const char *cgroup,
 		  int (*fn)(const char *cgroup, void *arg),
-- 
2.40.1

Re: [PATCH 1/2] mm, memcg: cg2 memory{.swap,}.peak write handlers

[Johannes Weiner]

Hi David,

thanks for pursuing this! A couple of comments below.

On Mon, Jul 22, 2024 at 07:55:53PM -0400, David Finkel wrote:
> @@ -1322,11 +1322,16 @@ PAGE_SIZE multiple when read back.
>  	reclaim induced by memory.reclaim.
>  
>    memory.peak
> -	A read-only single value file which exists on non-root
> -	cgroups.
> +	A read-write single value file which exists on non-root cgroups.
> +
> +	The max memory usage recorded for the cgroup and its descendants since
> +	either the creation of the cgroup or the most recent reset for that FD.
>  
> -	The max memory usage recorded for the cgroup and its
> -	descendants since the creation of the cgroup.
> +	A write of the string "reset" to this file resets it to the
> +	current memory usage for subsequent reads through the same
> +	file descriptor.
> +	Attempts to write any other non-empty string will return EINVAL
> +	(modulo leading and trailing whitespace).

Why not allow any write to reset? This makes it harder to use, and I'm
not sure accidental writes are a likely mistake to make.

> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 2150ca60394b..7001ed74e339 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -12,6 +12,7 @@
>  #include <linux/sched.h>
>  #include <linux/cpumask.h>
>  #include <linux/nodemask.h>
> +#include <linux/list.h>
>  #include <linux/rculist.h>
>  #include <linux/cgroupstats.h>
>  #include <linux/fs.h>
> @@ -855,4 +856,11 @@ static inline void cgroup_bpf_put(struct cgroup *cgrp) {}
>  
>  struct cgroup *task_get_cgroup1(struct task_struct *tsk, int hierarchy_id);
>  
> +struct memcg_peak_mem_ctx {
> +	long				local_watermark;
> +	struct list_head		peers;
> +};

Since this is generic cgroup code, and can be conceivably used by
other controllers, let's keep the naming generic as well. How about:

struct cgroup_of_peak {
	long			value;
	struct list_head	list;
};

cgroup-defs.h would be a better place for it.

> +struct memcg_peak_mem_ctx *memcg_extract_peak_mem_ctx(struct kernfs_open_file *of);

of_peak()

> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 030d34e9d117..cbc390234605 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -198,6 +198,11 @@ struct mem_cgroup {
>  	struct page_counter kmem;		/* v1 only */
>  	struct page_counter tcpmem;		/* v1 only */
>  
> +	/* lists of memcg peak watching contexts on swap and memory */
> +	struct list_head peak_memory_local_watermark_watchers;
> +	struct list_head peak_swap_local_watermark_watchers;
> +	spinlock_t swap_memory_peak_watchers_lock;

These names are too long. How about:

	/* Registered local usage peak watchers */
	struct list_head	memory_peaks;
	struct list_head	swap_peaks;
	spinlock_t		peaks_lock;

> diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> index 8cd858d912c4..06bb84218960 100644
> --- a/include/linux/page_counter.h
> +++ b/include/linux/page_counter.h
> @@ -26,6 +26,7 @@ struct page_counter {
>  	atomic_long_t children_low_usage;
>  
>  	unsigned long watermark;
> +	unsigned long local_watermark; /* track min of fd-local resets */
>  	unsigned long failcnt;
>  
>  	/* Keep all the read most fields in a separete cacheline. */
> @@ -78,7 +79,15 @@ int page_counter_memparse(const char *buf, const char *max,
>  
>  static inline void page_counter_reset_watermark(struct page_counter *counter)
>  {
> -	counter->watermark = page_counter_read(counter);
> +	unsigned long cur = page_counter_read(counter);

cur -> usage

> @@ -6907,12 +6912,109 @@ static u64 memory_current_read(struct cgroup_subsys_state *css,
>  	return (u64)page_counter_read(&memcg->memory) * PAGE_SIZE;
>  }
>  
> -static u64 memory_peak_read(struct cgroup_subsys_state *css,
> -			    struct cftype *cft)
> +inline int swap_memory_peak_show(
> +	struct seq_file *sf, void *v, bool swap_cg)
>  {

Leave inlining to the compiler. Just static int.

The name can be simply peak_show().

Customary coding style is to line wrap at the last parameter that
fits. Don't wrap if the line fits within 80 cols.

static int peak_show(struct seq_file *sf, void *v, ...,
		     ...)
{
	...
}

> +	struct cgroup_subsys_state *css = seq_css(sf);
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> +	struct page_counter *pc;
> +	struct kernfs_open_file *of = sf->private;
> +	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
> +	s64 fd_peak = ctx->local_watermark;
>  
> -	return (u64)memcg->memory.watermark * PAGE_SIZE;
> +	if (swap_cg)
> +		pc = &memcg->swap;
> +	else
> +		pc = &memcg->memory;
> +
> +	if (fd_peak == -1) {
> +		seq_printf(sf, "%llu\n", (u64)pc->watermark * PAGE_SIZE);
> +		return 0;
> +	}
> +
> +	s64 pc_peak = pc->local_watermark;
> +	s64 wm = fd_peak > pc_peak ? fd_peak : pc_peak;
> +
> +	seq_printf(sf, "%lld\n", wm * PAGE_SIZE);
> +	return 0;
> +}

As per Roman's feedback, don't mix decls and code.

You can simplify it by extracting css and memcg in the callers, then
pass the right struct page counter *pc directly.

That should eliminate most local variables as well.

static int peak_show(struct seq_file *sf, void *v, struct page_counter *pc)
{
	struct cgroup_of_peak *ofp = of_peak(sf->private);
	u64 peak;

	/* User wants global or local peak? */
	if (ofp->value == -1)
		peak = pc->watermark;
	else
		peak = max(ofp->value, pc->local_watermark);

	seq_printf(sf, "%lld\n", peak * PAGE_SIZE);
}

> +static int memory_peak_show(struct seq_file *sf, void *v)
> +{
> +	return swap_memory_peak_show(sf, v, false);

And then do:

	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));

	return peak_show(sf, v, &memcg->memory);

Then do the same with ... &memcg->swap.

> +inline ssize_t swap_memory_peak_write(
> +	struct kernfs_open_file *of,
> +	char *buf, size_t nbytes, loff_t off, bool swap_cg)
> +{

Same feedback as above. Please don't inline explicitly (unless it
really is measurably a performance improvement in a critical path),
and stick to surrounding coding style.

Here too, pass page_counter directly and save the branches.

> +	unsigned long cur;
> +	struct memcg_peak_mem_ctx *peer_ctx;
> +	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> +	struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
> +	struct page_counter *pc;
> +	struct list_head *watchers, *pos;
> +
> +	buf = strstrip(buf);
> +	/* Only allow "reset" to keep the API clear */
> +	if (strcmp(buf, "reset"))
> +		return -EINVAL;
> +
> +	if (swap_cg) {
> +		pc = &memcg->swap;
> +		watchers = &memcg->peak_swap_local_watermark_watchers;
> +	} else {
> +		pc = &memcg->memory;
> +		watchers = &memcg->peak_memory_local_watermark_watchers;
> +	}
> +
> +	spin_lock(&memcg->swap_memory_peak_watchers_lock);
> +
> +	page_counter_reset_local_watermark(pc);
> +	cur = pc->local_watermark;
> +
> +	list_for_each(pos, watchers) {

	list_for_each_entry()

> +		peer_ctx = list_entry(pos, typeof(*ctx), peers);
> +		if (cur > peer_ctx->local_watermark)
> +			peer_ctx->local_watermark = cur;
> +	}

I don't think this is quite right. local_peak could be higher than the
current usage when a new watcher shows up. The other watchers should
retain the higher local_peak, not the current usage.

> +
> +	if (ctx->local_watermark == -1)
> +		/* only append to the list if we're not already there */
> +		list_add_tail(&ctx->peers, watchers);
> +
> +	ctx->local_watermark = cur;

This makes me think that page_counter_reset_local_watermark() is not a
good helper. It obscures what's going on. Try without it.

AFAICS the list ordering doesn't matter, so keep it simple and use a
plain list_add().

	/*
	 * A new local peak is being tracked in pc->local_watermark.
	 * Save current local peak in all watchers.
	 */
	list_for_each_entry(pos, ...)
		if (pc->local_watermark > pos->value)
			pos->value = pc->local_watermark;

	pc->local_watermark = page_counter_read(pc);

	/* Initital write, register watcher */
	if (ofp->value == -1)
		list_add()

	ofp->value = pc->local_watermark;

> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index db20d6452b71..724d31508664 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -79,9 +79,22 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
>  		/*
>  		 * This is indeed racy, but we can live with some
>  		 * inaccuracy in the watermark.
> +		 *
> +		 * Notably, we have two watermarks to allow for both a globally
> +		 * visible peak and one that can be reset at a smaller scope.
> +		 *
> +		 * Since we reset both watermarks when the global reset occurs,
> +		 * we can guarantee that watermark >= local_watermark, so we
> +		 * don't need to do both comparisons every time.
> +		 *
> +		 * On systems with branch predictors, the inner condition should
> +		 * be almost free.
>  		 */
> -		if (new > READ_ONCE(c->watermark))
> -			WRITE_ONCE(c->watermark, new);
> +		if (new > READ_ONCE(c->local_watermark)) {
> +			WRITE_ONCE(c->local_watermark, new);
> +			if (new > READ_ONCE(c->watermark))
> +				WRITE_ONCE(c->watermark, new);
> +		}
>  	}
>  }
>  
> @@ -131,10 +144,23 @@ bool page_counter_try_charge(struct page_counter *counter,
>  		propagate_protected_usage(c, new);
>  		/*
>  		 * Just like with failcnt, we can live with some
> -		 * inaccuracy in the watermark.
> +		 * inaccuracy in the watermarks.
> +		 *
> +		 * Notably, we have two watermarks to allow for both a globally
> +		 * visible peak and one that can be reset at a smaller scope.
> +		 *
> +		 * Since we reset both watermarks when the global reset occurs,
> +		 * we can guarantee that watermark >= local_watermark, so we
> +		 * don't need to do both comparisons every time.
> +		 *
> +		 * On systems with branch predictors, the inner condition should
> +		 * be almost free.

		/* See comment in page_counter_charge() */

> diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
> index 432db923bced..1e2d46636a0c 100644
> --- a/tools/testing/selftests/cgroup/cgroup_util.c
> +++ b/tools/testing/selftests/cgroup/cgroup_util.c
> @@ -141,6 +141,16 @@ long cg_read_long(const char *cgroup, const char *control)
>  	return atol(buf);
>  }

This should be in patch #2.

Re: [PATCH 1/2] mm, memcg: cg2 memory{.swap,}.peak write handlers

[David Finkel]

Hi Johannes,

Thanks for the thorough review!

I'll send a rebased version addressing your comments in a moment.

On Tue, Jul 23, 2024 at 10:29 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> Hi David,
>
> thanks for pursuing this! A couple of comments below.
>
> On Mon, Jul 22, 2024 at 07:55:53PM -0400, David Finkel wrote:
> > @@ -1322,11 +1322,16 @@ PAGE_SIZE multiple when read back.
> >       reclaim induced by memory.reclaim.
> >
> >    memory.peak
> > -     A read-only single value file which exists on non-root
> > -     cgroups.
> > +     A read-write single value file which exists on non-root cgroups.
> > +
> > +     The max memory usage recorded for the cgroup and its descendants since
> > +     either the creation of the cgroup or the most recent reset for that FD.
> >
> > -     The max memory usage recorded for the cgroup and its
> > -     descendants since the creation of the cgroup.
> > +     A write of the string "reset" to this file resets it to the
> > +     current memory usage for subsequent reads through the same
> > +     file descriptor.
> > +     Attempts to write any other non-empty string will return EINVAL
> > +     (modulo leading and trailing whitespace).
>
> Why not allow any write to reset? This makes it harder to use, and I'm
> not sure accidental writes are a likely mistake to make.

Accepting any string is a rather wide interface, and since actually empty writes
get dropped before this handler is invoked, I think allowing any write
is more confusing.

I figured that a narrower interface is a better starting point, as
long as it's correctly
documented.

We can always widen it to all writes later, but we can't go the other way.

>
> > diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> > index 2150ca60394b..7001ed74e339 100644
> > --- a/include/linux/cgroup.h
> > +++ b/include/linux/cgroup.h
> > @@ -12,6 +12,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/cpumask.h>
> >  #include <linux/nodemask.h>
> > +#include <linux/list.h>
> >  #include <linux/rculist.h>
> >  #include <linux/cgroupstats.h>
> >  #include <linux/fs.h>
> > @@ -855,4 +856,11 @@ static inline void cgroup_bpf_put(struct cgroup *cgrp) {}
> >
> >  struct cgroup *task_get_cgroup1(struct task_struct *tsk, int hierarchy_id);
> >
> > +struct memcg_peak_mem_ctx {
> > +     long                            local_watermark;
> > +     struct list_head                peers;
> > +};
>
> Since this is generic cgroup code, and can be conceivably used by
> other controllers, let's keep the naming generic as well. How about:
>
> struct cgroup_of_peak {
>         long                    value;
>         struct list_head        list;
> };
>
> cgroup-defs.h would be a better place for it.
That is much less jarring.

>
> > +struct memcg_peak_mem_ctx *memcg_extract_peak_mem_ctx(struct kernfs_open_file *of);
>
> of_peak()
>
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 030d34e9d117..cbc390234605 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -198,6 +198,11 @@ struct mem_cgroup {
> >       struct page_counter kmem;               /* v1 only */
> >       struct page_counter tcpmem;             /* v1 only */
> >
> > +     /* lists of memcg peak watching contexts on swap and memory */
> > +     struct list_head peak_memory_local_watermark_watchers;
> > +     struct list_head peak_swap_local_watermark_watchers;
> > +     spinlock_t swap_memory_peak_watchers_lock;
>
> These names are too long. How about:
>
>         /* Registered local usage peak watchers */
>         struct list_head        memory_peaks;
>         struct list_head        swap_peaks;
>         spinlock_t              peaks_lock;
>
That's much better.
(sometimes when I'm unsure about a name, I default to a terrible, long name
with way too much information)

> > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> > index 8cd858d912c4..06bb84218960 100644
> > --- a/include/linux/page_counter.h
> > +++ b/include/linux/page_counter.h
> > @@ -26,6 +26,7 @@ struct page_counter {
> >       atomic_long_t children_low_usage;
> >
> >       unsigned long watermark;
> > +     unsigned long local_watermark; /* track min of fd-local resets */
> >       unsigned long failcnt;
> >
> >       /* Keep all the read most fields in a separete cacheline. */
> > @@ -78,7 +79,15 @@ int page_counter_memparse(const char *buf, const char *max,
> >
> >  static inline void page_counter_reset_watermark(struct page_counter *counter)
> >  {
> > -     counter->watermark = page_counter_read(counter);
> > +     unsigned long cur = page_counter_read(counter);
>
> cur -> usage
That's a better name.

>
> > @@ -6907,12 +6912,109 @@ static u64 memory_current_read(struct cgroup_subsys_state *css,
> >       return (u64)page_counter_read(&memcg->memory) * PAGE_SIZE;
> >  }
> >
> > -static u64 memory_peak_read(struct cgroup_subsys_state *css,
> > -                         struct cftype *cft)
> > +inline int swap_memory_peak_show(
> > +     struct seq_file *sf, void *v, bool swap_cg)
> >  {
>
> Leave inlining to the compiler. Just static int.
>
> The name can be simply peak_show().

Oh, right, I forgot that since this is local to the translation-unit
it doesn't need to be globally unique.

>
> Customary coding style is to line wrap at the last parameter that
> fits. Don't wrap if the line fits within 80 cols.
>
> static int peak_show(struct seq_file *sf, void *v, ...,
>                      ...)
> {
>         ...
> }
>
Oh yeah, I forgot to move those back to one line when I got rid of the
callback args
while addressing Roman's comments.
Thanks for pointing that out.

> > +     struct cgroup_subsys_state *css = seq_css(sf);
> >       struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> > +     struct page_counter *pc;
> > +     struct kernfs_open_file *of = sf->private;
> > +     struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
> > +     s64 fd_peak = ctx->local_watermark;
> >
> > -     return (u64)memcg->memory.watermark * PAGE_SIZE;
> > +     if (swap_cg)
> > +             pc = &memcg->swap;
> > +     else
> > +             pc = &memcg->memory;
> > +
> > +     if (fd_peak == -1) {
> > +             seq_printf(sf, "%llu\n", (u64)pc->watermark * PAGE_SIZE);
> > +             return 0;
> > +     }
> > +
> > +     s64 pc_peak = pc->local_watermark;
> > +     s64 wm = fd_peak > pc_peak ? fd_peak : pc_peak;
> > +
> > +     seq_printf(sf, "%lld\n", wm * PAGE_SIZE);
> > +     return 0;
> > +}
>
> As per Roman's feedback, don't mix decls and code.

Sorry, that one slipped through. (I've now fixed my syntax highlighting
to recognize s64 as a type)

>
> You can simplify it by extracting css and memcg in the callers, then
> pass the right struct page counter *pc directly.
>
> That should eliminate most local variables as well.
>
> static int peak_show(struct seq_file *sf, void *v, struct page_counter *pc)
> {
>         struct cgroup_of_peak *ofp = of_peak(sf->private);
>         u64 peak;
>
>         /* User wants global or local peak? */
>         if (ofp->value == -1)
>                 peak = pc->watermark;
>         else
>                 peak = max(ofp->value, pc->local_watermark);
>
>         seq_printf(sf, "%lld\n", peak * PAGE_SIZE);
> }
>
> > +static int memory_peak_show(struct seq_file *sf, void *v)
> > +{
> > +     return swap_memory_peak_show(sf, v, false);
>
> And then do:
>
>         struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));
>
>         return peak_show(sf, v, &memcg->memory);
>
> Then do the same with ... &memcg->swap.
That is much cleaner!
Thanks!
>
> > +inline ssize_t swap_memory_peak_write(
> > +     struct kernfs_open_file *of,
> > +     char *buf, size_t nbytes, loff_t off, bool swap_cg)
> > +{
>
> Same feedback as above. Please don't inline explicitly (unless it
> really is measurably a performance improvement in a critical path),
> and stick to surrounding coding style.
>
> Here too, pass page_counter directly and save the branches.
>
> > +     unsigned long cur;
> > +     struct memcg_peak_mem_ctx *peer_ctx;
> > +     struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> > +     struct memcg_peak_mem_ctx *ctx = memcg_extract_peak_mem_ctx(of);
> > +     struct page_counter *pc;
> > +     struct list_head *watchers, *pos;
> > +
> > +     buf = strstrip(buf);
> > +     /* Only allow "reset" to keep the API clear */
> > +     if (strcmp(buf, "reset"))
> > +             return -EINVAL;
> > +
> > +     if (swap_cg) {
> > +             pc = &memcg->swap;
> > +             watchers = &memcg->peak_swap_local_watermark_watchers;
> > +     } else {
> > +             pc = &memcg->memory;
> > +             watchers = &memcg->peak_memory_local_watermark_watchers;
> > +     }
> > +
> > +     spin_lock(&memcg->swap_memory_peak_watchers_lock);
> > +
> > +     page_counter_reset_local_watermark(pc);
> > +     cur = pc->local_watermark;
> > +
> > +     list_for_each(pos, watchers) {
>
>         list_for_each_entry()
Oh, I missed that one. Thanks!

>
> > +             peer_ctx = list_entry(pos, typeof(*ctx), peers);
> > +             if (cur > peer_ctx->local_watermark)
> > +                     peer_ctx->local_watermark = cur;
> > +     }
>
> I don't think this is quite right. local_peak could be higher than the
> current usage when a new watcher shows up. The other watchers should
> retain the higher local_peak, not the current usage.

Since we have to iterate over the list entries while holding the spinlock,
I think we avoid that kind of mismatch by keeping the assignments
inside that critical section as well. (and eliminate a few other races)

As-is, we get a snapshot value of the current usage and use that for
the rest of the function while holding the lock, so, those fd-local
watermarks will only change by assignments in this function.

On the other hand, pc->local_watermark may keep increasing, but that's fine.

>
> > +
> > +     if (ctx->local_watermark == -1)
> > +             /* only append to the list if we're not already there */
> > +             list_add_tail(&ctx->peers, watchers);
> > +
> > +     ctx->local_watermark = cur;
>
> This makes me think that page_counter_reset_local_watermark() is not a
> good helper. It obscures what's going on. Try without it.
>
> AFAICS the list ordering doesn't matter, so keep it simple and use a
> plain list_add().
>
>         /*
>          * A new local peak is being tracked in pc->local_watermark.
>          * Save current local peak in all watchers.
>          */
>         list_for_each_entry(pos, ...)
>                 if (pc->local_watermark > pos->value)
>                         pos->value = pc->local_watermark;
>
>         pc->local_watermark = page_counter_read(pc);
>
>         /* Initital write, register watcher */
>         if (ofp->value == -1)
>                 list_add()
>
>         ofp->value = pc->local_watermark;
>
> > diff --git a/mm/page_counter.c b/mm/page_counter.c
> > index db20d6452b71..724d31508664 100644
> > --- a/mm/page_counter.c
> > +++ b/mm/page_counter.c
> > @@ -79,9 +79,22 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
> >               /*
> >                * This is indeed racy, but we can live with some
> >                * inaccuracy in the watermark.
> > +              *
> > +              * Notably, we have two watermarks to allow for both a globally
> > +              * visible peak and one that can be reset at a smaller scope.
> > +              *
> > +              * Since we reset both watermarks when the global reset occurs,
> > +              * we can guarantee that watermark >= local_watermark, so we
> > +              * don't need to do both comparisons every time.
> > +              *
> > +              * On systems with branch predictors, the inner condition should
> > +              * be almost free.
> >                */
> > -             if (new > READ_ONCE(c->watermark))
> > -                     WRITE_ONCE(c->watermark, new);
> > +             if (new > READ_ONCE(c->local_watermark)) {
> > +                     WRITE_ONCE(c->local_watermark, new);
> > +                     if (new > READ_ONCE(c->watermark))
> > +                             WRITE_ONCE(c->watermark, new);
> > +             }
> >       }
> >  }
> >
> > @@ -131,10 +144,23 @@ bool page_counter_try_charge(struct page_counter *counter,
> >               propagate_protected_usage(c, new);
> >               /*
> >                * Just like with failcnt, we can live with some
> > -              * inaccuracy in the watermark.
> > +              * inaccuracy in the watermarks.
> > +              *
> > +              * Notably, we have two watermarks to allow for both a globally
> > +              * visible peak and one that can be reset at a smaller scope.
> > +              *
> > +              * Since we reset both watermarks when the global reset occurs,
> > +              * we can guarantee that watermark >= local_watermark, so we
> > +              * don't need to do both comparisons every time.
> > +              *
> > +              * On systems with branch predictors, the inner condition should
> > +              * be almost free.
>
>                 /* See comment in page_counter_charge() */
Even better!
>
> > diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
> > index 432db923bced..1e2d46636a0c 100644
> > --- a/tools/testing/selftests/cgroup/cgroup_util.c
> > +++ b/tools/testing/selftests/cgroup/cgroup_util.c
> > @@ -141,6 +141,16 @@ long cg_read_long(const char *cgroup, const char *control)
> >       return atol(buf);
> >  }
>
> This should be in patch #2.
:facepalm: that's what I get for trying to split this and send it out
at the end of the day.



-- 
David Finkel
Senior Principal Software Engineer, Core Services

[PATCH 2/2] mm, memcg: cg2 memory{.swap,}.peak write tests

[David Finkel]

Extend two existing tests to cover extracting memory usage through the
newly mutable memory.peak and memory.swap.peak handlers.

In particular, make sure to exercise adding and removing watchers with
overlapping lifetimes so the less-trivial logic gets tested.

Signed-off-by: David Finkel <davidf@vimeo.com>
---
 .../selftests/cgroup/test_memcontrol.c        | 227 +++++++++++++++++-
 1 file changed, 219 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 41ae8047b889..768f3e6c21af 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -161,12 +161,12 @@ static int alloc_pagecache_50M_check(const char *cgroup, void *arg)
 /*
  * This test create a memory cgroup, allocates
  * some anonymous memory and some pagecache
- * and check memory.current and some memory.stat values.
+ * and checks memory.current, memory.peak, and some memory.stat values.
  */
-static int test_memcg_current(const char *root)
+static int test_memcg_current_peak(const char *root)
 {
 	int ret = KSFT_FAIL;
-	long current;
+	long current, peak, peak_reset;
 	char *memcg;
 
 	memcg = cg_name(root, "memcg_test");
@@ -180,15 +180,109 @@ static int test_memcg_current(const char *root)
 	if (current != 0)
 		goto cleanup;
 
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak != 0)
+		goto cleanup;
+
 	if (cg_run(memcg, alloc_anon_50M_check, NULL))
 		goto cleanup;
 
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak < MB(50))
+		goto cleanup;
+
+	/*
+	 * We'll open a few FDs for the same memory.peak file to exercise the free-path
+	 * We need at least three to be closed in a different order than writes occurred to test
+	 * the linked-list handling.
+	 */
+	int peak_fd = cg_open(memcg, "memory.peak", O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (peak_fd == -1)
+		goto cleanup;
+
+	bool fd2_closed = false, fd3_closed = false, fd4_closed = false;
+	int peak_fd2 = cg_open(memcg, "memory.peak", O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (peak_fd2 == -1)
+		goto cleanup;
+
+	int peak_fd3 = cg_open(memcg, "memory.peak", O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (peak_fd3 == -1)
+		goto cleanup;
+
+	static const char reset_string[] = "reset\n";
+
+	peak_reset = write(peak_fd, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	peak_reset = write(peak_fd2, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	peak_reset = write(peak_fd3, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	/* Make sure a completely independent read isn't affected by our  FD-local reset above*/
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak < MB(50))
+		goto cleanup;
+
+	fd2_closed = true;
+	if (close(peak_fd2))
+		goto cleanup;
+
+	int peak_fd4 = cg_open(memcg, "memory.peak", O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (peak_fd4 == -1)
+		goto cleanup;
+
+	peak_reset = write(peak_fd4, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	peak = cg_read_long_fd(peak_fd);
+	if (peak > MB(30) || peak < 0)
+		goto cleanup;
+
 	if (cg_run(memcg, alloc_pagecache_50M_check, NULL))
 		goto cleanup;
 
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak < MB(50))
+		goto cleanup;
+
+	/* Make sure everything is back to normal */
+	peak = cg_read_long_fd(peak_fd);
+	if (peak < MB(50))
+		goto cleanup;
+
+	peak = cg_read_long_fd(peak_fd4);
+	if (peak < MB(50))
+		goto cleanup;
+
+	fd3_closed = true;
+	if (close(peak_fd3))
+		goto cleanup;
+
+	fd4_closed = true;
+	if (close(peak_fd4))
+		goto cleanup;
+
+
 	ret = KSFT_PASS;
 
 cleanup:
+	close(peak_fd);
+	if (!fd2_closed)
+		close(peak_fd2);
+	if (!fd3_closed)
+		close(peak_fd3);
+	if (!fd4_closed)
+		close(peak_fd4);
 	cg_destroy(memcg);
 	free(memcg);
 
@@ -817,13 +911,16 @@ static int alloc_anon_50M_check_swap(const char *cgroup, void *arg)
 
 /*
  * This test checks that memory.swap.max limits the amount of
- * anonymous memory which can be swapped out.
+ * anonymous memory which can be swapped out. Additionally, it verifies that
+ * memory.swap.peak reflects the high watermark and can be reset.
  */
-static int test_memcg_swap_max(const char *root)
+static int test_memcg_swap_max_peak(const char *root)
 {
 	int ret = KSFT_FAIL;
 	char *memcg;
-	long max;
+	long max, peak;
+
+	static const char reset_string[] = "reset\n";
 
 	if (!is_swap_enabled())
 		return KSFT_SKIP;
@@ -840,6 +937,45 @@ static int test_memcg_swap_max(const char *root)
 		goto cleanup;
 	}
 
+	int swap_peak_fd = cg_open(memcg, "memory.swap.peak",
+				   O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (swap_peak_fd == -1)
+		goto cleanup;
+
+	int mem_peak_fd = cg_open(memcg, "memory.peak", O_RDWR | O_APPEND | O_CLOEXEC);
+
+	if (mem_peak_fd == -1)
+		goto cleanup;
+
+	if (cg_read_long(memcg, "memory.swap.peak"))
+		goto cleanup;
+
+	if (cg_read_long_fd(swap_peak_fd))
+		goto cleanup;
+
+	/* switch the swap and mem fds into local-peak tracking mode*/
+	int peak_reset = write(swap_peak_fd, reset_string, sizeof(reset_string));
+
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	if (cg_read_long_fd(swap_peak_fd))
+		goto cleanup;
+
+	if (cg_read_long(memcg, "memory.peak"))
+		goto cleanup;
+
+	if (cg_read_long_fd(mem_peak_fd))
+		goto cleanup;
+
+	peak_reset = write(mem_peak_fd, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	if (cg_read_long_fd(mem_peak_fd))
+		goto cleanup;
+
 	if (cg_read_strcmp(memcg, "memory.max", "max\n"))
 		goto cleanup;
 
@@ -862,6 +998,61 @@ static int test_memcg_swap_max(const char *root)
 	if (cg_read_key_long(memcg, "memory.events", "oom_kill ") != 1)
 		goto cleanup;
 
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long(memcg, "memory.swap.peak");
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long_fd(mem_peak_fd);
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long_fd(swap_peak_fd);
+	if (peak < MB(29))
+		goto cleanup;
+
+	/*
+	 * open, reset and close the peak swap on another FD to make sure
+	 * multiple extant fds don't corrupt the linked-list
+	 */
+	peak_reset = cg_write(memcg, "memory.swap.peak", (char *)reset_string);
+	if (peak_reset)
+		goto cleanup;
+
+	peak_reset = cg_write(memcg, "memory.peak", (char *)reset_string);
+	if (peak_reset)
+		goto cleanup;
+
+	/* actually reset on the fds */
+	peak_reset = write(swap_peak_fd, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	peak_reset = write(mem_peak_fd, reset_string, sizeof(reset_string));
+	if (peak_reset != sizeof(reset_string))
+		goto cleanup;
+
+	peak = cg_read_long_fd(swap_peak_fd);
+	if (peak > MB(10))
+		goto cleanup;
+
+	/*
+	 * The cgroup is now empty, but there may be a page or two associated
+	 * with the open FD accounted to it.
+	 */
+	peak = cg_read_long_fd(mem_peak_fd);
+	if (peak > MB(1))
+		goto cleanup;
+
+	if (cg_read_long(memcg, "memory.peak") < MB(29))
+		goto cleanup;
+
+	if (cg_read_long(memcg, "memory.swap.peak") < MB(29))
+		goto cleanup;
+
 	if (cg_run(memcg, alloc_anon_50M_check_swap, (void *)MB(30)))
 		goto cleanup;
 
@@ -869,9 +1060,29 @@ static int test_memcg_swap_max(const char *root)
 	if (max <= 0)
 		goto cleanup;
 
+	peak = cg_read_long(memcg, "memory.peak");
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long(memcg, "memory.swap.peak");
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long_fd(mem_peak_fd);
+	if (peak < MB(29))
+		goto cleanup;
+
+	peak = cg_read_long_fd(swap_peak_fd);
+	if (peak < MB(19))
+		goto cleanup;
+
 	ret = KSFT_PASS;
 
 cleanup:
+	if (close(mem_peak_fd))
+		ret = KSFT_FAIL;
+	if (close(swap_peak_fd))
+		ret = KSFT_FAIL;
 	cg_destroy(memcg);
 	free(memcg);
 
@@ -1295,7 +1506,7 @@ struct memcg_test {
 	const char *name;
 } tests[] = {
 	T(test_memcg_subtree_control),
-	T(test_memcg_current),
+	T(test_memcg_current_peak),
 	T(test_memcg_min),
 	T(test_memcg_low),
 	T(test_memcg_high),
@@ -1303,7 +1514,7 @@ struct memcg_test {
 	T(test_memcg_max),
 	T(test_memcg_reclaim),
 	T(test_memcg_oom_events),
-	T(test_memcg_swap_max),
+	T(test_memcg_swap_max_peak),
 	T(test_memcg_sock),
 	T(test_memcg_oom_group_leaf_events),
 	T(test_memcg_oom_group_parent_events),
-- 
2.40.1

mm, memcg: cg2 memory{.swap,}.peak write handlers

[David Finkel]

This is an update of a previous series[1] addressing Johannes' comments,
and rebasing on top of linus's master.

Unfortunately, linus's master doesn't seem to be bootable at the
moment, so I haven't re-run the tests on this change yet. I'll see about
re-running everything in the morning. (root= resolution seems to be
failing both for x86-64 (in qemu) and usermode linux)

 Documentation/admin-guide/cgroup-v2.rst          |  26 ++--
 include/linux/cgroup-defs.h                      |   5 +
 include/linux/cgroup.h                           |   3 +
 include/linux/memcontrol.h                       |   5 +
 include/linux/page_counter.h                     |   6 +-
 kernel/cgroup/cgroup-internal.h                  |   2 +
 kernel/cgroup/cgroup.c                           |   7 ++
 mm/memcontrol.c                                  | 117 ++++++++++++++++--
 mm/page_counter.c                                |  30 +++--
 tools/testing/selftests/cgroup/cgroup_util.c     |  22 ++++
 tools/testing/selftests/cgroup/cgroup_util.h     |   2 +
 tools/testing/selftests/cgroup/test_memcontrol.c | 226 ++++++++++++++++++++++++++++++++--
 12 files changed, 416 insertions(+), 35 deletions(-)


[1]: https://lore.kernel.org/cgroups/20240722235554.2911971-1-davidf@vimeo.com/T/


Thanks for all the constructive comments and discussion!

David Finkel
Senior Principal Engineer, Core Services
Vimeo Inc.