On Wed, 2009-01-07 at 15:50 +0100, Frédéric Weisbecker wrote: > 2009/1/7 Peter Zijlstra <peterz@xxxxxxxxxxxxx>: > > 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@xxxxxxxxx> > > Reviewed-and-signed-off-by: Ingo Molnar <mingo@xxxxxxx> > > --- > 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. :-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
