Re: [PATCH v2 3/4] mm: workingset: skip the root memcg flush in workingset_test_recent()

[sashiko-bot@kernel.org]

> We can skip the rstat flush in workingset_test_recent() when the
> eviction memcg is the root memcg. lruvec_page_state() now reads the LRU
> sizes of the root memcg from the node counters, which are always up to
> date, and workingset_test_recent() only reads the eviction memcg's own
> lruvec. This avoids an expensive flush on the refault path, as flushing
> the root memcg flushes every memcg in the system.
> 
> For reclaim however, we still need to keep the flush (for now) since
> global and root reclaim also read the rstat stats of the child memcgs
> they scan, which the flush is needed for in order to keep up to date.
> 
> On an 80-CPU machine, a root flush took 69us on average with little
> cgroup activity and ~0.9ms with 128 memcgs actively allocating (wall
> time, including waiting for the rstat lock). Reading a node counter
> doesn't add any cost. It took the same amount of time as reading the
> rstat counter (~150ns, which includes bpftrace probe overhead).
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Sashiko has reviewed this patch and found no issues.

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