linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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Date:	Mon, 25 Nov 2013 13:01:13 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <davidlohr@...com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	"Norton, Scott J" <scott.norton@...com>, tom.vaden@...com,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	Jason Low <jason.low2@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Sat, 23 Nov 2013, Davidlohr Bueso wrote:
> On Sat, 2013-11-23 at 19:46 -0800, Linus Torvalds wrote:
> > On Sat, Nov 23, 2013 at 5:16 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > >
> > > Now the question is why we queue the waiter _AFTER_ reading the user
> > > space value. The comment in the code is pretty non sensical:
> > >
> > >    * On the other hand, we insert q and release the hash-bucket only
> > >    * after testing *uaddr.  This guarantees that futex_wait() will NOT
> > >    * absorb a wakeup if *uaddr does not match the desired values
> > >    * while the syscall executes.
> > >
> > > There is no reason why we cannot queue _BEFORE_ reading the user space
> > > value. We just have to dequeue in all the error handling cases, but
> > > for the fast path it does not matter at all.
> > >
> > > CPU 0                                   CPU 1
> > >
> > >     val = *futex;
> > >     futex_wait(futex, val);
> > >
> > >     spin_lock(&hb->lock);
> > >
> > >     plist_add(hb, self);
> > >     smp_wmb();
> > >
> > >     uval = *futex;
> > >                                         *futex = newval;
> > >                                         futex_wake();
> > >
> > >                                         smp_rmb();
> > >                                         if (plist_empty(hb))
> > >                                            return;
> > > ...
> > 
> > This would seem to be a nicer approach indeed, without needing the
> > extra atomics.
> 
> Yep, I think we can all agree that doing this optization without atomic
> ops is a big plus.
> 
> > 
> > Davidlohr, mind trying Thomas' approach?
> 
> I just took a quick look and it seems pretty straightforward, but not
> without some details to consider. We basically have to redo/reorder
> futex_wait_setup(), which checks that uval == val, and
> futex_wait_queue_me(), which adds the task to the list and blocks. Now,
> both futex_wait() and futex_wait_requeue_pi() have this logic, but since
> we don't use futex_wake() to wakeup tasks on pi futex_qs, I believe it's
> ok to only change futex_wait(), while the order of the uval checking
> doesn't matter for futex_wait_requeue_pi() so it can stay as is.

There is no mechanism which prevents a futex_wake() call on the inner
futex of the wait_requeue_pi mechanism. So no, we have to change both.

futexes are no place for believe. Either you understand them
completely or you just leave them alone.

Thanks,

	tglx
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