Re: [PATCH] mm: Do not allocate shrinker info with cgroup.memory=nokmem

[Roman Gushchin <roman.gushchin@linux.dev>]

Michal Koutný <mkoutny@suse.com> writes:

> There'd be no work for memcg-aware shrinkers when kernel memory is not
> accounted per cgroup, so we can skip allocating per memcg shrinker data.
> This saves some memory, avoids holding shrinker_mutex with O(nr_memcgs)
> and saves work in shrink_slab_memcg().
>
> Then there are SHRINKER_NONSLAB shrinkers which handle non-kernel memory
> so nokmem should not disable their per-memcg behavior. Such shrinkers
> (e.g.  deferred_split_shrinker) still need access to per-memcg data (see
> also commit 0a432dcbeb32e ("mm: shrinker: make shrinker not depend on
> memcg kmem")).
>
> The savings with this patch come on container hosts that create many
> superblocks (each with own shrinker) but tracking and processing
> per-memcg data is pointless with nokmem (shrink_slab_memcg() is
> partially guarded with !memcg_kmem_online already).
>
> The patch uses "boottime" predicate mem_cgroup_kmem_disabled() (not
> memcg_kmem_online()) to avoid mistakenly un-MEMCG_AWARE-ing shrinkers
> registered before first non-root memcg is mkdir'd.
>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!