linux-kernel - Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited process killing

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Date:   Thu, 11 Apr 2019 14:59:51 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        "yuzhoujian@...ichuxing.com" <yuzhoujian@...ichuxing.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Minchan Kim <minchan@...nel.org>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jann Horn <jannh@...gle.com>, linux-mm <linux-mm@...ck.org>,
        "lsf-pc@...ts.linux-foundation.org" 
        <lsf-pc@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited
 process killing

On Thu, Apr 11, 2019 at 2:45 PM Roman Gushchin <guro@...com> wrote:
>
> On Thu, Apr 11, 2019 at 10:09:06AM -0700, Suren Baghdasaryan wrote:
> > On Thu, Apr 11, 2019 at 8:33 AM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Apr 10, 2019 at 06:43:53PM -0700, Suren Baghdasaryan wrote:
> > > > Add new SS_EXPEDITE flag to be used when sending SIGKILL via
> > > > pidfd_send_signal() syscall to allow expedited memory reclaim of the
> > > > victim process. The usage of this flag is currently limited to SIGKILL
> > > > signal and only to privileged users.
> > >
> > > What is the downside of doing expedited memory reclaim?  ie why not do it
> > > every time a process is going to die?
>
> Hello, Suren!
>
> I also like the idea to reap always.
>
> > I think with an implementation that does not use/abuse oom-reaper
> > thread this could be done for any kill. As I mentioned oom-reaper is a
> > limited resource which has access to memory reserves and should not be
> > abused in the way I do in this reference implementation.
>
> In most OOM cases it doesn't matter that much which task to reap,
> so I don't think that reusing the oom-reaper thread is bad.
> It should be relatively easy to tweak in a way, that it won't
> wait for mmap_sem if there are other tasks waiting to be reaped.
> Also, the oom code add to the head of the list, and the expedited
> killing to the end, or something like this.
>
> The only think, if we're going to reap all tasks, we probably
> want to have a per-node oom_reaper thread.

Thanks for the ideas Roman. I'll take some time to digest the input
from everybody. What I heard from everyone is that we want this to be
a part of generic kill functionality which does not require a change
in userspace API.

> Thanks!
>
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