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Date:   Tue, 12 Mar 2019 17:48:57 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <christian@brauner.io>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, devel@driverdev.osuosl.org,
        linux-mm <linux-mm@kvack.org>, Tim Murray <timmurray@google.com>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
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On Tue 12-03-19 09:37:41, Sultan Alsawaf wrote:
> On Tue, Mar 12, 2019 at 09:05:32AM +0100, Michal Hocko wrote:
> > The only way to control the OOM behavior pro-actively is to throttle
> > allocation speed. We have memcg high limit for that purpose. Along with
> > PSI, I can imagine a reasonably working user space early oom
> > notifications and reasonable acting upon that.
> 
> The issue with pro-active memory management that prompted me to create this was
> poor memory utilization. All of the alternative means of reclaiming pages in the
> page allocator's slow path turn out to be very useful for maximizing memory
> utilization, which is something that we would have to forgo by relying on a
> purely pro-active solution. I have not had a chance to look at PSI yet, but
> unless a PSI-enabled solution allows allocations to reach the same point as when
> the OOM killer is invoked (which is contradictory to what it sets out to do),
> then it cannot take advantage of all of the alternative memory-reclaim means
> employed in the slowpath, and will result in killing a process before it is
> _really_ necessary.

If you really want to reach the real OOM situation then you can very
well rely on the in-kernel OOM killer. The only reason you want a
customized oom killer is the tasks clasification. And that is a
different story. User space hints on the victim selection has been a
topic for quite while. It never get to any conclusion as interested
parties have always lost an interest because it got hairy quickly.

> > If you design is relies on the speed of killing then it is fundamentally
> > flawed AFAICT. You cannot assume anything about how quickly a task dies.
> > It might be blocked in an uninterruptible sleep or performin an
> > operation which takes some time. Sure, oom_reaper might help here but
> > still.
> 
> In theory we could instantly zap any process that is not trapped in the kernel
> at the time that the OOM killer is invoked without any consequences though, no?

No, this is not so simple. Have a look at the oom_reaper and hops it has
to go through.
-- 
Michal Hocko
SUSE Labs