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Date:   Tue, 5 May 2020 12:40:31 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>,
        Greg Thelen <gthelen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Cgroups <cgroups@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] memcg: oom: ignore oom warnings from memory.max
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On Tue, May 05, 2020 at 08:35:45AM -0700, Shakeel Butt wrote:
> On Tue, May 5, 2020 at 8:27 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Mon, May 04, 2020 at 12:23:51PM -0700, Shakeel Butt wrote:
> > > On Mon, May 4, 2020 at 9:06 AM Michal Hocko <mhocko@kernel.org> wrote:
> > > > I really hate to repeat myself but this is no different from a regular
> > > > oom situation.
> > >
> > > Conceptually yes there is no difference but there is no *divine
> > > restriction* to not make a difference if there is a real world
> > > use-case which would benefit from it.
> >
> > I would wholeheartedly agree with this in general.
> >
> > However, we're talking about the very semantics that set memory.max
> > apart from memory.high: triggering OOM kills to enforce the limit.
> >
> > > > when the kernel cannot act and mentions that along with the
> > > > oom report so that whoever consumes that information can debug or act on
> > > > that fact.
> > > >
> > > > Silencing the oom report is simply removing a potentially useful
> > > > aid to debug further a potential problem.
> > >
> > > *Potentially* useful for debugging versus actually beneficial for
> > > "sweep before tear down" use-case. Also I am not saying to make "no
> > > dumps for memory.max when no eligible tasks" a set in stone rule. We
> > > can always reevaluate when such information will actually be useful.
> > >
> > > Johannes/Andrew, what's your opinion?
> >
> > I still think that if you want to sweep without triggering OOMs,
> > memory.high has the matching semantics.
> >
> > As you pointed out, it doesn't work well for foreign charges, but that
> > is more of a limitation in the implementation than in the semantics:
> >
> >         /*
> >          * If the hierarchy is above the normal consumption range, schedule
> >          * reclaim on returning to userland.  We can perform reclaim here
> >          * if __GFP_RECLAIM but let's always punt for simplicity and so that
> >          * GFP_KERNEL can consistently be used during reclaim.  @memcg is
> >          * not recorded as it most likely matches current's and won't
> >          * change in the meantime.  As high limit is checked again before
> >          * reclaim, the cost of mismatch is negligible.
> >          */
> >
> > Wouldn't it be more useful to fix that instead? It shouldn't be much
> > of a code change to do sync reclaim in try_charge().
> 
> Sync reclaim would really simplify the remote charging case. Though
> should sync reclaim only be done for remote charging or for all?

I would do it for all __GFP_RECLAIM callers, no need to special case
remote charging unnecessarily IMO.

We can do both the reclaim as well as the penalty throttling
synchronously, i.e. all of mem_cgroup_handle_over_high(). And then
punt to the userspace resume when we either didn't reclaim or are
still over the threshold after reclaim.

Btw we should probably kick off high_work rather than set userspace
resume on foreign charges, right? Otherwise we may not reclaim at all
when we push a group over its limit from outside (and in a
!__GFP_RECLAIM context).