linux-kernel - Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 07 Jan 2009 15:58:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning

On Wed, 2009-01-07 at 15:50 +0100, Frédéric Weisbecker wrote:
> 2009/1/7 Peter Zijlstra <peterz@...radead.org>:
> > Change mutex contention behaviour such that it will sometimes busy wait on
> > acquisition - moving its behaviour closer to that of spinlocks.
> >
> > This concept got ported to mainline from the -rt tree, where it was originally
> > implemented for rtmutexes by Steven Rostedt, based on work by Gregory Haskins.
> >
> > Testing with Ingo's test-mutex application (http://lkml.org/lkml/2006/1/8/50)
> > gave a 8% boost for VFS scalability on my testbox:
> >
> >  # echo MUTEX_SPIN > /debug/sched_features
> >  # ./test-mutex V 16 10
> >  2 CPUs, running 16 parallel test-tasks.
> >  checking VFS performance.
> >
> >  avg ops/sec:                74910
> >
> >  # echo NO_MUTEX_SPIN > /debug/sched_features
> >  # ./test-mutex V 16 10
> >  2 CPUs, running 16 parallel test-tasks.
> >  checking VFS performance.
> >
> >  avg ops/sec:                68804
> >
> > The key criteria for the busy wait is that the lock owner has to be running on
> > a (different) cpu. The idea is that as long as the owner is running, there is a
> > fair chance it'll release the lock soon, and thus we'll be better off spinning
> > instead of blocking/scheduling.
> >
> > Since regular mutexes (as opposed to rtmutexes) do not atomically track the
> > owner, we add the owner in a non-atomic fashion and deal with the races in
> > the slowpath.
> >
> > Furthermore, to ease the testing of the performance impact of this new code,
> > there is means to disable this behaviour runtime (without having to reboot
> > the system), when scheduler debugging is enabled (CONFIG_SCHED_DEBUG=y),
> > by issuing the following command:
> >
> >  # echo NO_MUTEX_SPIN > /debug/sched_features
> >
> > This command re-enables spinning again (this is also the default):
> >
> >  # echo MUTEX_SPIN > /debug/sched_features
> >
> > There's also a few new statistic fields in /proc/sched_debug
> > (available if CONFIG_SCHED_DEBUG=y and CONFIG_SCHEDSTATS=y):
> >
> >  # grep mtx /proc/sched_debug
> >  .mtx_spin                      : 2387
> >  .mtx_sched                     : 2283
> >  .mtx_spin                      : 1277
> >  .mtx_sched                     : 1700
> >
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Reviewed-and-signed-off-by: Ingo Molnar <mingo@...e.hu>
> > ---

> Sorry I haven't read all the previous talk about the older version.
> But it is possible that, in hopefully rare cases, you enter
> mutex_spin_or_schedule
> multiple times, and try to spin for the same lock each of these times.
> 
> For each of the above break,
> 
> _if you exit the spin because the mutex is unlocked, and someone else
> grab it before you
> _ or simply the owner changed...
> 
> then you will enter again in mutex_spin_or_schedule, you have some chances that
> rq->curr == the new owner, and then you will spin again.
> And this situation can almost really make you behave like a spinlock...

You understand correctly, that is indeed possible.

> Shouldn't it actually try only one time to spin, and if it calls again
> mutex_spin_or_schedule()
> then it would be better to schedule()  ?

I don't know, maybe code it up and find a benchmark where it makes a
difference. :-)
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