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Date:   Tue, 12 Mar 2019 09:05:32 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Sultan Alsawaf <sultan@...neltoast.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, devel@...verdev.osuosl.org,
        linux-mm <linux-mm@...ck.org>, Tim Murray <timmurray@...gle.com>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

On Mon 11-03-19 15:15:35, Suren Baghdasaryan wrote:
> On Mon, Mar 11, 2019 at 1:46 PM Sultan Alsawaf <sultan@...neltoast.com> wrote:
> >
> > On Mon, Mar 11, 2019 at 01:10:36PM -0700, Suren Baghdasaryan wrote:
> > > The idea seems interesting although I need to think about this a bit
> > > more. Killing processes based on failed page allocation might backfire
> > > during transient spikes in memory usage.
> >
> > This issue could be alleviated if tasks could be killed and have their pages
> > reaped faster. Currently, Linux takes a _very_ long time to free a task's memory
> > after an initial privileged SIGKILL is sent to a task, even with the task's
> > priority being set to the highest possible (so unwanted scheduler preemption
> > starving dying tasks of CPU time is not the issue at play here). I've
> > frequently measured the difference in time between when a SIGKILL is sent for a
> > task and when free_task() is called for that task to be hundreds of
> > milliseconds, which is incredibly long. AFAIK, this is a problem that LMKD
> > suffers from as well, and perhaps any OOM killer implementation in Linux, since
> > you cannot evaluate effect you've had on memory pressure by killing a process
> > for at least several tens of milliseconds.
> 
> Yeah, killing speed is a well-known problem which we are considering
> in LMKD. For example the recent LMKD change to assign process being
> killed to a cpuset cgroup containing big cores cuts the kill time
> considerably. This is not ideal and we are thinking about better ways
> to expedite the cleanup process.

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.

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.
-- 
Michal Hocko
SUSE Labs

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