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

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Date:   Mon, 24 Jul 2023 11:31:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: memcg: use rstat for non-hierarchical stats

On Wed, 19 Jul 2023 17:46:13 +0000 Yosry Ahmed <yosryahmed@...gle.com> 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.