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 10:09:06 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, mhocko@...e.com,
        David Rientjes <rientjes@...gle.com>,
        yuzhoujian@...ichuxing.com,
        Souptick Joarder <jrdr.linux@...il.com>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        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,
        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 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?

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.
While there might be downsides that I don't know of, I'm not sure it's
required to hurry every kill's memory reclaim. I think there are cases
when resource deallocation is critical, for example when we kill to
relieve resource shortage and there are kills when reclaim speed is
not essential. It would be great if we can identify urgent cases
without userspace hints, so I'm open to suggestions that do not
involve additional flags.