[RFC] alternative mechanism to skip memcg kmem allocations

[Glauber Costa <glommer@parallels.com>]

Since Kame expressed the wish to see a context-based method to skip
accounting for caches, I came up with the following proposal for
your appreciation.

It basically works in the same way as preempt_disable()/preempt_enable():
By marking a region under which all allocations will be accounted
to the root memcg.

I basically see two main advantages of it:

 * No need to clutter the code with *_noaccount functions; they could
   become specially widespread if we needed to skip accounting for
   kmalloc variants like track, zalloc, etc.
 * Works with other caches, not only kmalloc; specially interesting
   since during cache creation we touch things like cache_cache,
   that could very well we wrapped inside a noaccount region.

However:

 * It touches task_struct
 * It is harder to keep drivers away from using it. With
   kmalloc_no_account we could simply not export it. Here, one can
   always set this in the task_struct...

Let me know what you think of it.

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Christoph Lameter <cl@linux.com>
CC: Pekka Enberg <penberg@cs.helsinki.fi>
CC: Michal Hocko <mhocko@suse.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Suleiman Souhlal <suleiman@google.com>
---
 include/linux/sched.h |    1 +
 mm/memcontrol.c       |   34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 81a173c..516a9fe 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1613,6 +1613,7 @@ struct task_struct {
 		unsigned long nr_pages;	/* uncharged usage */
 		unsigned long memsw_nr_pages; /* uncharged mem+swap usage */
 	} memcg_batch;
+	int memcg_kmem_skip_account;
 #endif
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 	atomic_t ptrace_bp_refcnt;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8c7c404..833f4cd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -479,6 +479,33 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
 EXPORT_SYMBOL(tcp_proto_cgroup);
 #endif /* CONFIG_INET */
 
+static void memcg_stop_kmem_account(void)
+{
+	struct task_struct *p;
+
+	if (!current->mm)
+		return;
+
+	p = rcu_dereference(current->mm->owner);
+	if (p) {
+		task_lock(p);
+		p->memcg_kmem_skip_account = true;
+	}
+}
+
+static void memcg_start_kmem_account(void)
+{
+	struct task_struct *p;
+
+	if (!current->mm)
+		return;
+
+	p = rcu_dereference(current->mm->owner);
+	if (p) {
+		p->memcg_kmem_skip_account = false;
+		task_unlock(p);
+	}
+}
 char *mem_cgroup_cache_name(struct mem_cgroup *memcg, struct kmem_cache *cachep)
 {
 	char *name;
@@ -541,7 +568,9 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	if (new_cachep)
 		goto out;
 
+	memcg_stop_kmem_account();
 	new_cachep = kmem_cache_dup(memcg, cachep);
+	memcg_start_kmem_account();
 
 	if (new_cachep == NULL) {
 		new_cachep = cachep;
@@ -634,7 +663,9 @@ static void memcg_create_cache_enqueue(struct mem_cgroup *memcg,
 	if (!css_tryget(&memcg->css))
 		return;
 
+	memcg_stop_kmem_account();
 	cw = kmalloc(sizeof(struct create_work), GFP_NOWAIT);
+	memcg_start_kmem_account();
 	if (cw == NULL) {
 		css_put(&memcg->css);
 		return;
@@ -678,6 +709,9 @@ struct kmem_cache *__mem_cgroup_get_kmem_cache(struct kmem_cache *cachep,
 	VM_BUG_ON(idx == -1);
 
 	p = rcu_dereference(current->mm->owner);
+	if (p->memcg_kmem_skip_account)
+		return cachep;
+
 	memcg = mem_cgroup_from_task(p);
 
 	if (!mem_cgroup_kmem_enabled(memcg))
-- 
1.7.7.6

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