linux-kernel - Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

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Date:   Tue, 5 May 2020 12:57:34 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>,
        Greg Thelen <gthelen@...gle.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: oom: ignore oom warnings from memory.max

On Tue, May 05, 2020 at 08:03:17AM -0700, Shakeel Butt wrote:
> On Tue, May 5, 2020 at 12:13 AM Michal Hocko <mhocko@...nel.org> wrote:
> > So instead of potentially removing a useful information can we focus on
> > the remote charging side of the problem and deal with it in a sensible
> > way? That would make memory.high usable for your usecase and I still
> > believe that this is what you should be using in the first place.
> 
> We talked about this at LSFMM'19 and I think the decision was to not
> fix high reclaim for remote memcg until it will be an actual issue. I
> suppose now we can treat it as an actual issue.
> 
> There are a couple of open questions:
> 1) Should the remote chargers be throttled and do the high reclaim?

I would say it depends on the caller. We may need both: a centralized
charger like a kthread that does work on behalf of many cgroups should
not be throttled, or we risk priority inversions - maybe not even held
up for reclaim, although that would need some thinking about
memory.max as well - whether remote charges should be allowed to
overrun memory.max temporarily, and we punt all reclaim from such
contexts to a worker. (Preferrably, in the future, with all the cpu
cycles of that worker accounted to the cgroup. We want that for
kswapd-style background reclaim as well).

We do similar things with IO for example, when offloading IO to a
central kthread. The kthread is allowed to proceed past a cgroup's
allowance in order to make forward progress, and the cgroup will be
forced to pay up retroactively when it comes back for more of the
resource.