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

From: Daniel Colascione <dancol@google.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	David Rientjes <rientjes@google.com>,
	yuzhoujian@didichuxing.com,
	Souptick Joarder <jrdr.linux@gmail.com>,
	Roman Gushchin <guro@fb.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Shakeel Butt <shakeelb@google.com>,
	Christian Brauner <christian@brauner.io>,
	Minchan Kim <minchan@kernel.org>,
	Tim Murray <timmurray@google.com>,
	Daniel Colascione <dancol@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Jann Horn <jannh@google.com>, linux-mm <linux-mm@kvack.org>,
	lsf-pc@lists.linux-foundation.org,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>
Subject: Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited process killing
Date: Thu, 11 Apr 2019 10:33:32 -0700	[thread overview]
Message-ID: <CAKOZuetFU4tXE27bxA86zzDfNSCbw83p8fPxfkQ_d_Em0C04Sg@mail.gmail.com> (raw)
In-Reply-To: <CAJuCfpGQ8c-OCws-zxZyqKGy1CfZpjxDKMH__qAm5FFXBcnWOw@mail.gmail.com>

On Thu, Apr 11, 2019 at 10:09 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Apr 11, 2019 at 8:33 AM Matthew Wilcox <willy@infradead.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.

I was imagining a PI-ish approach where we'd reap in case an RT
process was waiting on the death of some other process. I'd still
prefer the API I proposed in the other message because it gets the
kernel out of the business of deciding what the right signal is. I'm a
huge believer in "mechanism, not policy".