Re: [PATCH] mm: memcg: use rstat for non-hierarchical stats

[Yosry Ahmed]

On Mon, Jul 24, 2023 at 11:31 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 19 Jul 2023 17:46:13 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> > Currently, memcg uses rstat to maintain hierarchical stats. The rstat
> > framework keeps track of which cgroups have updates on which cpus.
> >
> > For non-hierarchical stats, as memcg moved to rstat, they are no longer
> > readily available as counters. Instead, the percpu counters for a given
> > stat need to be summed to get the non-hierarchical stat value. This
> > causes a performance regression when reading non-hierarchical stats on
> > kernels where memcg moved to using rstat. This is especially visible
> > when reading memory.stat on cgroup v1. There are also some code paths
> > internal to the kernel that read such non-hierarchical stats.
> >
> > It is inefficient to iterate and sum counters in all cpus when the rstat
> > framework knows exactly when a percpu counter has an update. Instead,
> > maintain cpu-aggregated non-hierarchical counters for each stat. During
> > an rstat flush, keep those updated as well. When reading
> > non-hierarchical stats, we no longer need to iterate cpus, we just need
> > to read the maintainer counters, similar to hierarchical stats.
> >
> > A caveat is that we now 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 /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
> >
>
> I'll queue this for some testing, pending reviewer input, please.

Thanks Andrew!

I am doing extra testing of my own as well as of now. Will report back
if anything interesting pops up.