linux-kernel - Re: [PATCH] memcg: introduce per-memcg reclaim interface

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Date:   Tue, 22 Sep 2020 13:02:47 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Greg Thelen <gthelen@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: introduce per-memcg reclaim interface

On Tue, Sep 22, 2020 at 12:09 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 22-09-20 11:10:17, Shakeel Butt wrote:
> > On Tue, Sep 22, 2020 at 9:55 AM Michal Hocko <mhocko@...e.com> wrote:
> [...]
> > > Last but not least the memcg
> > > background reclaim is something that should be possible without a new
> > > interface.
> >
> > So, it comes down to adding more functionality/semantics to
> > memory.high or introducing a new simple interface. I am fine with
> > either of one but IMO convoluted memory.high might have a higher
> > maintenance cost.
>
> One idea would be to schedule a background worker (which work on behalf
> on the memcg) to do the high limit reclaim with high limit target as
> soon as the high limit is reached. There would be one work item for each
> memcg. Userspace would recheck the high limit on return to the userspace
> and do the reclaim if the excess is larger than a threshold, and sleep
> as the fallback.
>
> Excessive consumers would get throttled if the background work cannot
> keep up with the charge pace and most of them would return without doing
> any reclaim because there is somebody working on their behalf - and is
> accounted for that.
>
> The semantic of high limit would be preserved IMHO because high limit is
> actively throttled. Where that work is done shouldn't matter as long as
> it is accounted properly and memcg cannot outsource all the work to the
> rest of the system.
>
> Would something like that (with many details to be sorted out of course)
> be feasible?

This is exactly how our "per-memcg kswapd" works. The missing piece is
how to account the background worker (it is a kernel work thread)
properly as what we discussed before. You mentioned such work is WIP
in earlier email of this thread, I think once this is done the
per-memcg background worker could be supported easily.

>
> If we do not want to change the existing semantic of high and want a new
> api then I think having another limit for the background reclaim then
> that would make more sense to me. It would resemble the global reclaim
> and kswapd model and something that would be easier to reason about.
> Comparing to echo $N > reclaim which might mean to reclaim any number
> pages around N.
> --
> Michal Hocko
> SUSE Labs