Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup

From: Davidlohr Bueso
Date: Thu Jan 02 2014 - 15:59:27 EST

On Thu, 2014-01-02 at 11:23 -0800, Linus Torvalds wrote:
> On Thu, Jan 2, 2014 at 7:05 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> >
> > In futex_wake() there is clearly no point in taking the hb->lock if we know
> > beforehand that there are no tasks to be woken.
> Btw, I think we could optimize this a bit further for the wakeup case.
> wake_futex() does a get_task_struct(p)/put_task_struct(p) around its
> actual waking logic, and I don't think that's necessary. The task
> structures are RCU-delayed, and the task cannot go away until the
> "q->lock_ptr = NULL" afaik, so you could replace that atomic inc/dec
> with just a RCU read region.

I had originally explored making the whole plist thing more rcu aware
but never got to anything worth sharing. What you say does make a lot of
sense, however, I haven't been able to see any actual improvements. It
doesn't hurt however, so I'd have no problem adding such patch to the

> Maybe it's not a big deal ("wake_up_state()" ends up getting the task
> struct pi_lock anyway, so it's not like we can avoid toucing the task
> structure), but I'm getting the feeling that we're doing a lot of
> unnecessary work here.

I passed this idea through my wakeup measuring program and didn't notice
hardly any difference, just noise, even for large amounts of futexes.
I believe that peterz's idea of lockless batch wakeups is the next step
worth looking into for futexes -- even though the spurious wakeup
problem can become a real pain.


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