Re: [PATCH v3 3/5] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 13, 2021 at 09:20:25PM -0400, Waiman Long wrote:
> Before the new slab memory controller with per object byte charging,
> charging and vmstat data update happen only when new slab pages are
> allocated or freed. Now they are done with every kmem_cache_alloc()
> and kmem_cache_free(). This causes additional overhead for workloads
> that generate a lot of alloc and free calls.
> 
> The memcg_stock_pcp is used to cache byte charge for a specific
> obj_cgroup to reduce that overhead. To further reducing it, this patch
> makes the vmstat data cached in the memcg_stock_pcp structure as well
> until it accumulates a page size worth of update or when other cached
> data change.
> 
> On a 2-socket Cascade Lake server with instrumentation enabled and this
> patch applied, it was found that about 17% (946796 out of 5515184) of the
> time when __mod_obj_stock_state() is called leads to an actual call to
> mod_objcg_state() after initial boot. When doing parallel kernel build,
> the figure was about 16% (21894614 out of 139780628). So caching the
> vmstat data reduces the number of calls to mod_objcg_state() by more
> than 80%.

Right, but mod_objcg_state() is itself already percpu-cached. What's
the benefit of avoiding calls to it with another percpu cache?