Messages in this thread | | | Date | Thu, 2 Jan 2014 11:23:05 -0800 | Subject | Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup | From | Linus Torvalds <> |
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On Thu, Jan 2, 2014 at 7:05 AM, Davidlohr Bueso <davidlohr@hp.com> 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.
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.
This only triggers for the actual case of having a task to wake up, though, so not your particular test load.
Linus
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