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From:   Christian Brauner <christian@brauner.io>
Date:   Tue, 12 Mar 2019 19:50:56 +0100
Message-ID: <CAHrFyr7wJsNAQ_NPAsBDA-g7useeetgJ-MBod40SQAmf08sXJw@mail.gmail.com>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
To:     Tim Murray <timmurray@google.com>
Cc:     Sultan Alsawaf <sultan@kerneltoast.com>,
        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>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-drivers <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>
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On Tue, Mar 12, 2019 at 7:43 PM Tim Murray <timmurray@google.com> wrote:
>
> 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.

Fwiw, I am working on this and have send a PR for inclusion in 5.1:
https://lore.kernel.org/lkml/20190312135245.27591-1-christian@brauner.io/
There's also a tree to track this work.

Christian