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From:   Tim Murray <timmurray@google.com>
Date:   Tue, 12 Mar 2019 11:43:13 -0700
Message-ID: <CAEe=Sx=MxzBB46WxuwHTQcocBkx1UW+fmVOa3VWv_eUferzVYw@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>,
        Suren Baghdasaryan <surenb@google.com>,
        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>
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On Tue, Mar 12, 2019 at 10:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote:
>
> On Tue, Mar 12, 2019 at 10:17:43AM -0700, Tim Murray wrote:
> > Knowing whether a SIGKILL'd process has finished reclaiming is as far
> > as I know not possible without something like procfds. That's where
> > the 100ms timeout in lmkd comes in. lowmemorykiller and lmkd both
> > attempt to wait up to 100ms for reclaim to finish by checking for the
> > continued existence of the thread that received the SIGKILL, but this
> > really means that they wait up to 100ms for the _thread_ to finish,
> > which doesn't tell you anything about the memory used by that process.
> > If those threads terminate early and lowmemorykiller/lmkd get a signal
> > to kill again, then there may be two processes competing for CPU time
> > to reclaim memory. That doesn't reclaim any faster and may be an
> > unnecessary kill.
> > ...
> > - offer a way to wait for process termination so lmkd can tell when
> > reclaim has finished and know when killing another process is
> > appropriate
>
> Should be pretty easy with something like this:

Yeah, that's in the spirit of what I was suggesting, but there are lot
of edge cases around how to get that data out efficiently and PID
reuse (it's a real issue--often the Android apps that are causing
memory pressure are also constantly creating/destroying threads).

I believe procfds or a similar mechanism will be a good solution to this.