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Date:   Thu, 14 Mar 2019 20:43:28 -0700
From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Tim Murray <timmurray@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        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>,
        Christian Brauner <christian@brauner.io>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>,
        kernel-team <kernel-team@android.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
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On Thu, Mar 14, 2019 at 10:54:48PM -0400, Joel Fernandes wrote:
> I'm not sure if that makes much semantic sense for how the signal handling is
> supposed to work. Imagine a parent sends SIGKILL to its child, and then does
> a wait(2). Because the SIGKILL blocks in your idea, then the wait cannot
> execute, and because the wait cannot execute, the zombie task will not get
> reaped and so the SIGKILL senders never gets unblocked and the whole thing
> just gets locked up. No? I don't know it just feels incorrect.

Block until the victim becomes a zombie instead.

> Further, in your idea adding stuff to task_struct will simply bloat it - when
> this task can easily be handled using eBPF without making any kernel changes.
> Either by probing sched_process_free or sched_process_exit tracepoints.
> Scheduler maintainers generally frown on adding stuff to task_struct
> pointlessly there's a good reason since bloating it effects the performance
> etc, and something like this would probably never be ifdef'd out behind a
> CONFIG.

Adding something to task_struct is just the easiest way to test things for
experimentation. This can be avoided in my suggestion by passing the pointer to
a completion via the relevant functions, and then completing it at the time the
victim transitions to a zombie state. I understand it's possible to use eBPF for
this, but it seems kind of messy since this functionality is something that I
think others would want provided by the kernel (i.e., anyone using PSI to
implement their own OOM killer daemon similar to LMKD).

Thanks,
Sultan