linux-kernel - Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention

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Date:	Wed, 14 Sep 2011 17:51:38 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention

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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