linux-kernel - Re: [RFC PATCH] mm: memcontrol: don't account swap failures not due to cgroup limits

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Date:   Mon, 6 Feb 2023 17:18:43 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [RFC PATCH] mm: memcontrol: don't account swap failures not due
 to cgroup limits

On Thu, Feb 02, 2023 at 10:30:40AM -0800, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > b) Only count cgroup swap events when they are actually due to a
> >    cgroup's own limit. Exclude failures that are due to physical swap
> >    shortage or other system-level conditions (like !THP_SWAP). Also
> >    count them at the level where the limit is configured, which may be
> >    above the local cgroup that holds the page-to-be-swapped.
> >
> >    This is in line with how memory.swap.high, memory.high and
> >    memory.max events are counted.
> >
> >    However, it's a change in documented behavior.
> 
> This option makes sense to me, but I can't speak to the change of
> documented behavior. However, looking at the code, it seems like if we do this
> the "max" & "fail" counters become effectively the same. "fail" would
> not provide much value then.
> 
> I wonder if it makes sense to have both, and clarify that "fail" -
> "max" would be non-limit based failures (e.g. ran out of swap space),
> or would this cause confusion as to whether those non-limit failures
> were transient (THP fallback) or eventual?

I somewhat second this.

Perhaps, could the patch (and arguments) be split in two:
1) count .max events on respective limit's level (other limits consistency),
2) redefine (remove?) memory.swap.fail events?

Michal

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