On Wed, Jul 26, 2023 at 12:29:04AM +0000, Yosry Ahmed wrote: > Currently, memcg uses rstat to maintain 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. > > For non-hierarchical stats, we do not maintain counters. Instead, the global? > percpu counters for a given stat need to be summed to get the > non-hierarchical stat value. The original implementation did the same. > At some point before rstat, non-hierarchical counters were introduced by > commit a983b5ebee57 ("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 815744d75152 ("mm: memcontrol: don't batch > updates of local VM stats and events"). See [1] for more detailed > history. > > Kernel versions in between a983b5ebee57 & 815744d75152 (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 a stats flush before reading need? > 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 /dev/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 > > [1]https://lore.kernel.org/lkml/20230725201811.GA1231514@xxxxxxxxxxx/ > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx> Thank you!