Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake_affine(.sync=true)

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 26, 2013 at 03:55:55AM -0700, Paul Turner wrote:
> > +               /*
> > +                * Don't bother with select_idle_sibling() in the case of a sync wakeup
> > +                * where we know the only running task will soon go-away. Going
> > +                * through select_idle_sibling will only lead to pointless ping-pong.
> > +                */
> > +               if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1 &&
> 
> I've long thought of trying something like this.
> 
> I like the intent but I'd go a step further in that I think we want to
> also implicitly extract WF_SYNC itself.

I have vague memories of actually trying something like that a good
number of years ago.. sadly that's all I remember about it.

> What we really then care about is predicting the overlap associated
> with userspace synchronization objects, typically built on top of
> futexes.  Unfortunately the existence/use of per-thread futexes
> reduces how much state you could usefully associate with the futex.
> One approach might be to hash (with some small saturating counter)
> against rip.  But this gets more complicated quite quickly.

Why would you need per object storage? To further granulate the
predicted overlap? instead of having one per task, you have one per
object?