linux-kernel - Re: Expensive memory.stat + cpu.stat reads

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Date:   Mon, 10 Jul 2023 16:23:45 -0700
From:   Ivan Babrou <ivan@...udflare.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     cgroups@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        kernel-team <kernel-team@...udflare.com>,
        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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Expensive memory.stat + cpu.stat reads

On Mon, Jul 10, 2023 at 7:44 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> Hello.
>
> On Fri, Jun 30, 2023 at 04:22:28PM -0700, Ivan Babrou <ivan@...udflare.com> wrote:
> > As you might've noticed from the output, splitting the loop into two
> > makes the code run 10x faster.
>
> That is curious.
>
> > We're running Linux v6.1 (the output is from v6.1.25) with no patches
> > that touch the cgroup or mm subsystems, so you can assume vanilla
> > kernel.
>
> Have you watched for this on older kernels too?

We've been on v6.1 for quite a while now, but it's possible that we
weren't paying enough attention before to notice.

> > I am happy to try out patches or to do some tracing to help understand
> > this better.
>
> I see in your reproducer you tried swapping order of controllers
> flushed.
> Have you also tried flushing same controller twice (in the inner loop)?
> (Despite the expectation is that it shouldn't be different from half the
> scenario where ran two loops.)

Same controller twice is fast (whether it's mem + mem or cpu + cpu):

warm-up
completed: 17.24s [manual / cpu-stat + mem-stat]
completed:  1.02s [manual / mem-stat+mem-stat]
completed:  0.59s [manual / cpu-stat+cpu-stat]
completed:  0.44s [manual / mem-stat]
completed:  0.16s [manual / cpu-stat]
running
completed: 14.32s [manual / cpu-stat + mem-stat]
completed:  1.25s [manual / mem-stat+mem-stat]
completed:  0.42s [manual / cpu-stat+cpu-stat]
completed:  0.12s [manual / mem-stat]
completed:  0.50s [manual / cpu-stat]