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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Mon, 11 Mar 2019 15:15:35 -0700
Message-ID: <CAJuCfpGpBxofTT-ANEEY+dFCSdwkQswox3s8Uk9Eq0BnK9i0iA@mail.gmail.com>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
To:     Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <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>
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On Mon, Mar 11, 2019 at 1:46 PM Sultan Alsawaf <sultan@kerneltoast.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.

> > AFAIKT the biggest issue with using this approach in userspace is that
> > it's not practically implementable without heavy in-kernel support.
> > How to implement such interaction between kernel and userspace would
> > be an interesting discussion which I would be happy to participate in.
>
> You could signal a lightweight userspace process that has maximum scheduler
> priority and have it kill the tasks it'd like.

This what LMKD currently is - a userspace RT process.
My point was that this page allocation queue that you implemented
can't be implemented in userspace, at least not without extensive
communication with kernel.

> Thanks,
> Sultan

Thanks,
Suren.