Currently, memcg uses rstat to maintain aggregated hierarchical stats. 
Counters are maintained for hierarchical stats at each memcg.  Rstat
tracks which cgroups have updates on which cpus to keep those counters
fresh on the read-side.

Non-hierarchical stats are currently not covered by rstat.  Their per-cpu
counters are summed up on every read, which is expensive.  The original
implementation did the same.  At some point before rstat, non-hierarchical
aggregated counters were introduced by commit a983b5eb ("mm:
memcontrol: fix excessive complexity in memory.stat reporting").  However,
those counters were updated on the performance critical write-side, which
caused regressions, so they were later removed by commit 815744d7
("mm: memcontrol: don't batch updates of local VM stats and events").  See
[1] for more detailed history.

Kernel versions in between a983b5eb & 815744d7 (a year and a half)
enjoyed cheap reads of non-hierarchical stats, specifically on cgroup v1. 
When moving to more recent kernels, a performance regression for reading
non-hierarchical stats is observed.

Now that we have rstat, we know exactly which percpu counters have updates
for each stat.  We can maintain non-hierarchical counters again, making
reads much more efficient, without affecting the performance critical
write-side.  Hence, add non-hierarchical (i.e local) counters for the
stats, and extend rstat flushing to keep those up-to-date.

A caveat is that we now need a stats flush before reading
local/non-hierarchical stats through {memcg/lruvec}_page_state_local() or
memcg_events_local(), where we previously only needed a flush to read
hierarchical stats.  Most contexts reading non-hierarchical stats are
already doing a flush, add a flush to the only missing context in
count_shadow_nodes().

With this patch, reading memory.stat from 1000 memcgs is 3x faster on a
machine with 256 cpus on cgroup v1:

 # for i in $(seq 1000); do mkdir /sys/fs/cgroup/memory/cg$i; done
 # time cat /sys/fs/cgroup/memory/cg*/memory.stat > /dev/null
 real	 0m0.125s
 user	 0m0.005s
 sys	 0m0.120s

After:
 real	 0m0.032s
 user	 0m0.005s
 sys	 0m0.027s

To make sure there are no regressions on cgroup v2, I ran an artificial
reclaim/refault stress test [2] that creates (NR_CPUS * 2) cgroups,
assigns them limits, runs a worker process in each cgroup that allocates
tmpfs memory equal to quadruple the limit (to invoke reclaim
continuously), and then reads back the entire file (to invoke refaults). 
All workers are run in parallel, and zram is used as a swapping backend. 
Both reclaim and refault have conditional stats flushing.  I ran this on a
machine with 112 cpus, once on mm-unstable, and once on mm-unstable with
this patch reverted.

(1) A few runs without this patch:

 # time ./stress_reclaim_refault.sh
 real 0m9.949s
 user 0m0.496s
 sys 14m44.974s

 # time ./stress_reclaim_refault.sh
 real 0m10.049s
 user 0m0.486s
 sys 14m55.791s

 # time ./stress_reclaim_refault.sh
 real 0m9.984s
 user 0m0.481s
 sys 14m53.841s

(2) A few runs with this patch:

 # time ./stress_reclaim_refault.sh
 real 0m9.885s
 user 0m0.486s
 sys 14m48.753s

 # time ./stress_reclaim_refault.sh
 real 0m9.903s
 user 0m0.495s
 sys 14m48.339s

 # time ./stress_reclaim_refault.sh
 real 0m9.861s
 user 0m0.507s
 sys 14m49.317s

No regressions are observed with this patch. There is actually a very
slight improvement. If I have to guess, maybe it's because we avoid
the percpu loop in count_shadow_nodes() when calling
lruvec_page_state_local(), but I could not prove this using perf, it's
probably in the noise.

[1] https://lore.kernel.org/lkml/20230725201811.GA1231514@cmpxchg.org/
[2] https://lore.kernel.org/lkml/CAJD7tkb17x=qwoO37uxyYXLEUVp15BQKR+Xfh7Sg9Hx-wTQ_=w@mail.gmail.com/

Link: https://lkml.kernel.org/r/20230803185046.1385770-1-yosryahmed@google.com
Link: https://lkml.kernel.org/r/20230726153223.821757-2-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>