Re: [PATCH RFC] reduce runqueue lock contention

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-05-20 at 16:48 -0400, Chris Mason wrote:
> 
> This is more of a starting point than a patch, but it is something I've
> been meaning to look at for a long time.  Many different workloads end
> up hammering very hard on try_to_wake_up, to the point where the
> runqueue locks dominate CPU profiles.

Right, so one of the things that I considered was to make p->state an
atomic_t and replace the initial stage of try_to_wake_up() with
something like:

int try_to_wake_up(struct task *p, unsigned int mask, wake_flags)
{
  int state = atomic_read(&p->state);

  do {
    if (!(state & mask))
      return 0;

    state = atomic_cmpxchg(&p->state, state, TASK_WAKING);
  } while (state != TASK_WAKING);

  /* do this pending queue + ipi thing */

  return 1;
}

Also, I think we might want to put that atomic single linked list thing
into some header (using atomic_long_t or so), because I have a similar
thing living in kernel/perf_event.c, that needs to queue things from NMI
context.

The advantage of doing basically the whole enqueue on the remote cpu is
less cacheline bouncing of the runqueue structures.