Messages in this thread | | | Subject | Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention | From | Peter Zijlstra <> | Date | Wed, 14 Sep 2011 17:51:38 +0200 |
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On Wed, 2011-09-14 at 08:46 -0700, Darren Hart wrote: > > On 09/14/2011 06:30 AM, Peter Zijlstra wrote: > > Use the brand spanking new wake_list to delay the futex wakeups until > > after we've released the hash bucket locks. This avoids the newly > > woken tasks from immediately getting stuck on the hb lock. > > > > This is esp. painful on -rt, where the hb lock is preemptible. > > Nice! > > Have you run this through the functional and performance tests from > futextest? Looks like I should also add a multiwake test to really > showcase this.
Not more functional than booting, but a very similar patch used to live in 33-rt.. I lost the use-case we had that led to that patch, for -rt it made a huge difference because we endlessly scheduled back and forth between the waker and the wakee bouncing on the hb lock.
> If you don't have it local I can setup a github repository for futextest > until korg is back.... or do the testing myself... right.
Right, I don't think I have futextest, or I might, I'd have to dig around a bit.
> > @@ -988,7 +986,7 @@ futex_wake(u32 __user *uaddr, unsigned i > > if (!(this->bitset & bitset)) > > continue; > > > > - wake_futex(this); > > + wake_futex(&wake_list, this); > > > I guess this is OK. wake_futex_pi will always be one task I believe, so > the list syntax might confuse newcomers... Would it make sense to have a > wake_futex_list() call? Thinking outloud...
To what purpose? Even delaying a single wakeup until after we release the hb lock is useful. On it matters even on !-rt since the woken task can wake on another cpu and then spin on hb-lock.
> > @@ -1437,6 +1441,7 @@ static int futex_requeue(u32 __user *uad > > put_futex_key(&key2); > > out_put_key1: > > put_futex_key(&key1); > > + wake_up_list(&wake_list, TASK_NORMAL); > > out: > > if (pi_state != NULL) > > free_pi_state(pi_state); > > > > > > I _think_ requeue_pi is in the clear here as it uses > requeue_pi_wake_futex, which calls wake_up_state directly. Still, some > testing with futextest functional/futex_requeue_pi is in order.
Ah, right, that might want frobbing too..
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