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From:   Joel Fernandes <joelaf@google.com>
Date:   Mon, 29 Oct 2018 12:45:25 -0700
Message-ID: <CAJWu+opR5bU7b0ux5GZjO-Wu+=2F8sARke-E_y4PpH0XSubx9g@mail.gmail.com>
Subject: Re: [RFC PATCH] Minimal non-child process exit notification support
To:     Daniel Colascione <dancol@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>
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On Mon, Oct 29, 2018 at 10:53 AM Daniel Colascione <dancol@google.com> wrote:
>
> This patch adds a new file under /proc/pid, /proc/pid/exithand.
> Attempting to read from an exithand file will block until the
> corresponding process exits, at which point the read will successfully
> complete with EOF.  The file descriptor supports both blocking
> operations and poll(2). It's intended to be a minimal interface for
> allowing a program to wait for the exit of a process that is not one
> of its children.
>
> Why might we want this interface? Android's lmkd kills processes in
> order to free memory in response to various memory pressure
> signals. It's desirable to wait until a killed process actually exits
> before moving on (if needed) to killing the next process. Since the
> processes that lmkd kills are not lmkd's children, lmkd currently
> lacks a way to wait for a proces to actually die after being sent
> SIGKILL; today, lmkd resorts to polling the proc filesystem pid

Any idea why it needs to wait and then send SIGKILL? Why not do
SIGKILL and look for errno == ESRCH in a loop with a delay.

> entry. This interface allow lmkd to give up polling and instead block
> and wait for process death.

Can we use ptrace(2) for the exit notifications? I am assuming you
already though about it but I'm curious what is the reason this is
better.

thanks,

-Joel