Re: [PATCH] mm: memcontrol: flush percpu vmevents before releasing memcg

[Michal Hocko <mhocko@kernel.org>]

On Mon 12-08-19 16:37:54, Roman Gushchin wrote:
> Similar to vmstats, percpu caching of local vmevents leads to an
> accumulation of errors on non-leaf levels. This happens because
> some leftovers may remain in percpu caches, so that they are
> never propagated up by the cgroup tree and just disappear into
> nonexistence with on releasing of the memory cgroup.
> 
> To fix this issue let's accumulate and propagate percpu vmevents
> values before releasing the memory cgroup similar to what we're
> doing with vmstats.
> 
> Since on cpu hotplug we do flush percpu vmstats anyway, we can
> iterate only over online cpus.
> 
> Fixes: 42a300353577 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6d2427abcc0c..249187907339 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3459,6 +3459,25 @@ static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only)
>  	}
>  }
>  
> +static void memcg_flush_percpu_vmevents(struct mem_cgroup *memcg)
> +{
> +	unsigned long events[NR_VM_EVENT_ITEMS];
> +	struct mem_cgroup *mi;
> +	int cpu, i;
> +
> +	for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> +		events[i] = 0;
> +
> +	for_each_online_cpu(cpu)
> +		for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> +			events[i] += raw_cpu_read(
> +				memcg->vmstats_percpu->events[i]);
> +
> +	for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> +		for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> +			atomic_long_add(events[i], &mi->vmevents[i]);
> +}
> +
>  static void memcg_offline_kmem(struct mem_cgroup *memcg)
>  {
>  	struct cgroup_subsys_state *css;
> @@ -4860,10 +4879,11 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
>  	int node;
>  
>  	/*
> -	 * Flush percpu vmstats to guarantee the value correctness
> +	 * Flush percpu vmstats and vmevents to guarantee the value correctness
>  	 * on parent's and all ancestor levels.
>  	 */
>  	memcg_flush_percpu_vmstats(memcg, false);
> +	memcg_flush_percpu_vmevents(memcg);
>  	for_each_node(node)
>  		free_mem_cgroup_per_node_info(memcg, node);
>  	free_percpu(memcg->vmstats_percpu);
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs