Messages in this thread | | | Subject | Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup | From | Davidlohr Bueso <> | Date | Thu, 02 Jan 2014 12:59:17 -0800 |
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On Thu, 2014-01-02 at 11:23 -0800, Linus Torvalds wrote: > 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.
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 lot.
> > 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.
Thanks, Davidlohr
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