Re: [RFC v2] prctl: prctl(PR_SET_IDLE, PR_IDLE_MODE_KILLME), for stateless idle loops

[Shawn Landden <slandden@gmail.com>]

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On Fri, Nov 3, 2017 at 2:09 AM, Michal Hocko <mhocko@kernel.org> wrote:

> On Thu 02-11-17 23:35:44, Shawn Landden wrote:
> > It is common for services to be stateless around their main event loop.
> > If a process sets PR_SET_IDLE to PR_IDLE_MODE_KILLME then it
> > signals to the kernel that epoll_wait() and friends may not complete,
> > and the kernel may send SIGKILL if resources get tight.
> >
> > See my systemd patch: https://github.com/shawnl/systemd/tree/prctl
> >
> > Android uses this memory model for all programs, and having it in the
> > kernel will enable integration with the page cache (not in this
> > series).
> >
> > 16 bytes per process is kinda spendy, but I want to keep
> > lru behavior, which mem_score_adj does not allow. When a supervisor,
> > like Android's user input is keeping track this can be done in
> user-space.
> > It could be pulled out of task_struct if an cross-indexing additional
> > red-black tree is added to support pid-based lookup.
>
> This is still an abuse and the patch is wrong. We really do have an API
> to use I fail to see why you do not use it.
>
When I looked at wait_queue_head_t it was 20 byes.

>
> [...]
> > @@ -1018,6 +1060,24 @@ bool out_of_memory(struct oom_control *oc)
> >                       return true;
> >       }
> >
> > +     /*
> > +      * Check death row for current memcg or global.
> > +      */
> > +     l = oom_target_get_queue(current);
> > +     if (!list_empty(l)) {
> > +             struct task_struct *ts = list_first_entry(l,
> > +                             struct task_struct, se.oom_target_queue);
> > +
> > +             pr_debug("Killing pid %u from EPOLL_KILLME death row.",
> > +                      ts->pid);
> > +
> > +             /* We use SIGKILL instead of the oom killer
> > +              * so as to cleanly interrupt ep_poll()
> > +              */
> > +             send_sig(SIGKILL, ts, 1);
> > +             return true;
> > +     }
>
> Still not NUMA aware and completely backwards. If this is a memcg OOM
> then it is _memcg_ to evaluate not the current. The oom might happen up
> the hierarchy due to hard limit.
>
> But still, you should be very clear _why_ the existing oom tuning is not
> appropropriate and we can think of a way to hanle it better but cramming
> the oom selection this way is simply not acceptable.
> --
> Michal Hocko
> SUSE Labs
>

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