[PATCH v4] cgroup2: New memory.max.effective like cgroup1 hierarchical_memory_limit

[PATCH v4] cgroup2: New memory.max.effective like cgroup1 hierarchical_memory_limit

[Jan Kratochvil (Azul)]

Implement cgroup2 "memory.max.effective" and "memory.swap.max.effective" files
similar to existing "cpuset.cpus.effective".

cgroup1 (by function memcg1_stat_format) contains two lines
	hierarchical_memory_limit %llu
	hierarchical_memsw_limit %llu

which are useful for userland to easily and performance-wise find out the
effective cgroup limits being applied. Otherwise userland has to
open+read+close the file "memory.max" and/or "memory.swap.max" in multiple
parent directories of a nested cgroup.

For cgroup1 it was implemented by:
	memcg: show real limit under hierarchy mode
	https://github.com/torvalds/linux/commit/fee7b548e6f2bd4bfd03a1a45d3afd593de7d5e9
	Date:   Wed Jan 7 18:08:26 2009 -0800

But for cgroup2 it has been missing so far.

v4:
Adjust the commit message.
v3:
memory.stat fields -> *.max.effective separate files suggested by Michal Koutny
v2:
hierarchical_memsw_limit -> hierarchical_swap_limit fix found by Waiman Long
v1:
hierarchical_memory_limit && hierarchical_memsw_limit in memory.stat


Signed-off-by: Jan Kratochvil (Azul) <jkratochvil@azul.com>

 mm/memcontrol.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1ed40f9d3..8c4cb5f60 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6845,6 +6845,19 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static int memory_max_effective_show(struct seq_file *m, void *v)
+{
+	unsigned long memory;
+	struct mem_cgroup *mi;
+
+	/* Hierarchical information */
+	memory = PAGE_COUNTER_MAX;
+	for (mi = mem_cgroup_from_seq(m); mi; mi = parent_mem_cgroup(mi))
+		memory = min(memory, READ_ONCE(mi->memory.max));
+
+	return seq_puts_memcg_tunable(m, memory);
+}
+
 /*
  * Note: don't forget to update the 'samples/cgroup/memcg_event_listener'
  * if any new events become available.
@@ -7038,6 +7051,11 @@ static struct cftype memory_files[] = {
 		.seq_show = memory_max_show,
 		.write = memory_max_write,
 	},
+	{
+		.name = "max.effective",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = memory_max_effective_show,
+	},
 	{
 		.name = "events",
 		.flags = CFTYPE_NOT_ON_ROOT,
@@ -8040,6 +8058,19 @@ static ssize_t swap_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static int swap_max_effective_show(struct seq_file *m, void *v)
+{
+	unsigned long swap;
+	struct mem_cgroup *mi;
+
+	/* Hierarchical information */
+	swap = PAGE_COUNTER_MAX;
+	for (mi = mem_cgroup_from_seq(m); mi; mi = parent_mem_cgroup(mi))
+		swap = min(swap, READ_ONCE(mi->swap.max));
+
+	return seq_puts_memcg_tunable(m, swap);
+}
+
 static int swap_events_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
@@ -8072,6 +8103,11 @@ static struct cftype swap_files[] = {
 		.seq_show = swap_max_show,
 		.write = swap_max_write,
 	},
+	{
+		.name = "swap.max.effective",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = swap_max_effective_show,
+	},
 	{
 		.name = "swap.peak",
 		.flags = CFTYPE_NOT_ON_ROOT,

Re: [PATCH v4] cgroup2: New memory.max.effective like cgroup1 hierarchical_memory_limit

[Michal Koutný]

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Hello.

I have some advise about patch desription and only nitpick about
implementation.

On Wed, Feb 14, 2024 at 05:56:20AM +0800, "Jan Kratochvil (Azul)" <jkratochvil@azul.com> wrote:
> which are useful for userland to easily and performance-wise find out the
> effective cgroup limits being applied.

I think this is a weak reasoning for in-kernel implementation.
It may be faster but how often do you need to read that limit?

> Otherwise userland has to

Userland is at loss when running inside cgroupns with limited ancestors
out of the cgroupns. Thus I think this is the reason why kernel can
provide such an effective value. (And be subject line of the commit, I
think bringing up cgroup1 is misleading.)

> For cgroup1 it was implemented by:
> 	memcg: show real limit under hierarchy mode
> 	https://github.com/torvalds/linux/commit/fee7b548e6f2bd4bfd03a1a45d3afd593de7d5e9
> 	Date:   Wed Jan 7 18:08:26 2009 -0800

FTR, commits are usually referenced more concisely like
	fee7b548e6f2 ("memcg: show real limit under hierarchy mode") 
(the document Waiman linked previously has some tips how to get this
from git).

> +static int memory_max_effective_show(struct seq_file *m, void *v)
> +{
> +	unsigned long memory;
> +	struct mem_cgroup *mi;
> +
> +	/* Hierarchical information */
> +	memory = PAGE_COUNTER_MAX;
> +	for (mi = mem_cgroup_from_seq(m); mi; mi = parent_mem_cgroup(mi))
> +		memory = min(memory, READ_ONCE(mi->memory.max));

root_mem_cgroup is never charged (thus limited), so you can terminate
the iteration on !mem_cgroup_is_root(mi), one level earlier

> +static int swap_max_effective_show(struct seq_file *m, void *v)
> +{
> +	unsigned long swap;
> +	struct mem_cgroup *mi;
> +
> +	/* Hierarchical information */
> +	swap = PAGE_COUNTER_MAX;
> +	for (mi = mem_cgroup_from_seq(m); mi; mi = parent_mem_cgroup(mi))
> +		swap = min(swap, READ_ONCE(mi->swap.max));

ditto

HTH,
Michal

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