mm/memcg: cache vmstat data in percpu memcg_stock_pcp

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.
Caching the vmstat data in the per-cpu stock eliminates two writes to
non-hot cachelines for memcg specific as well as memcg-lruvecs specific
vmstat data by a write to a hot local stock cacheline.

On a 2-socket Cascade Lake server with instrumentation enabled and this
patch applied, it was found that about 20% (634400 out of 3243830) of the
time when mod_objcg_state() is called leads to an actual call to
__mod_objcg_state() after initial boot.  When doing parallel kernel build,
the figure was about 17% (24329265 out of 142512465).  So caching the
vmstat data reduces the number of calls to __mod_objcg_state() by more
than 80%.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Waiman Long <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Chris Down <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Masayoshi Mizuma <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Cc: Yafang Shao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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