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

[Daniel Colascione]

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