Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup

Johannes Weiner
2020-08-11T15:28:45Z

On Tue, Jun 23, 2020 at 11:45:14AM -0700, Roman Gushchin wrote:
> Memory cgroups are using large chunks of percpu memory to store vmstat
> data. Yet this memory is not accounted at all, so in the case when there
> are many (dying) cgroups, it's not exactly clear where all the memory is.
>
> Because the size of memory cgroup internal structures can dramatically
> exceed the size of object or page which is pinning it in the memory, it's
> not a good idea to simple ignore it. It actually breaks the isolation
> between cgroups.
>
> Let's account the consumed percpu memory to the parent cgroup.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Dennis Zhou <dennis@kernel.org>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

This makes sense, and the accounting is in line with how we track and
distribute child creation quotas (cgroup.max.descendants and
cgroup.max.depth) up the cgroup tree.

I have one minor comment that isn't a dealbreaker for me:

> @@ -5069,13 +5069,15 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
> if (!pn)
> return 1;
>
> - pn->lruvec_stat_local = alloc_percpu(struct lruvec_stat);
> + pn->lruvec_stat_local = alloc_percpu_gfp(struct lruvec_stat,
> + GFP_KERNEL_ACCOUNT);
> if (!pn->lruvec_stat_local) {
> kfree(pn);
> return 1;
> }
>
> - pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
> + pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
> + GFP_KERNEL_ACCOUNT);
> if (!pn->lruvec_stat_cpu) {
> free_percpu(pn->lruvec_stat_local);
> kfree(pn);
> @@ -5149,11 +5151,13 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> goto fail;
> }
>
> - memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
> + memcg->vmstats_local = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> + GFP_KERNEL_ACCOUNT);
> if (!memcg->vmstats_local)
> goto fail;
>
> - memcg->vmstats_percpu = alloc_percpu(struct memcg_vmstats_percpu);
> + memcg->vmstats_percpu = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> + GFP_KERNEL_ACCOUNT);
> if (!memcg->vmstats_percpu)
> goto fail;
>
> @@ -5202,7 +5206,9 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
> struct mem_cgroup *memcg;
> long error = -ENOMEM;
>
> + memalloc_use_memcg(parent);
> memcg = mem_cgroup_alloc();
> + memalloc_unuse_memcg();

The disconnect between 1) requesting accounting and 2) which cgroup to
charge is making me uneasy. It makes mem_cgroup_alloc() a bit of a
handgrenade, because accounting to the current task is almost
guaranteed to be wrong if the use_memcg() annotation were to get lost
in a refactor or not make it to a new caller of the function.

The saving grace is that mem_cgroup_alloc() is pretty unlikely to be
used elsewhere. And pretending it's an independent interface would be
overengineering. But how about the following in mem_cgroup_alloc() and
alloc_mem_cgroup_per_node_info() to document that caller relationship:

/* We charge the parent cgroup, never the current task */
WARN_ON_ONCE(!current->active_memcg);