Re: [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping

Davidlohr Bueso
2015-01-27T21:57:31Z

On Tue, 2015-01-27 at 18:34 +0100, Peter Zijlstra wrote:
> On Sun, Jan 25, 2015 at 11:36:08PM -0800, Davidlohr Bueso wrote:
> > When blocking , we incur in multiple barriers when setting the
> > task's uninterruptable state. This is particularly bad when the
> > lock keeps getting stolen from the task trying to acquire the sem.
> > These changes propose delaying setting the task's new state until
> > we are sure that calling schedule is inevitable.
> >
> > This implies that we do the trylock and active check (both basically
> > ->counter checks) as TASK_RUNNING. For the trylock we hold the wait
> > lock with interrupts disabled, so no risk there. And for the active
> > check, the window for which we could get interrupted is quite small
> > and makes no tangible difference.
> >
> > This patch increases Unixbench's 'execl' throughput by 25% on a 40
> > core machine.
> >
> > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> > ---
> > kernel/locking/rwsem-xadd.c | 20 +++++++++++++++-----
> > 1 file changed, 15 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> > index 18a50da..88b3468 100644
> > --- a/kernel/locking/rwsem-xadd.c
> > +++ b/kernel/locking/rwsem-xadd.c
> > @@ -459,17 +459,27 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
> > count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> >
> > /* wait until we successfully acquire the lock */
> > while (true) {
> > if (rwsem_try_write_lock(count, sem))
> > break;
> > +
> > + __set_current_state(TASK_UNINTERRUPTIBLE);
> > raw_spin_unlock_irq(&sem->wait_lock);
> >
> > + /*
> > + * When there are active locks after we wake up,
> > + * the lock was probably stolen from us. Thus,
> > + * go immediately back to sleep and avoid taking
> > + * the wait_lock.
> > + */
> > + while (true) {
> > schedule();
> > +
> > + count = READ_ONCE(sem->count);
> > + if (!(count & RWSEM_ACTIVE_MASK))
> > + break;
> > + __set_current_state(TASK_UNINTERRUPTIBLE);
> > + }
>
> So its late and I'm not seeing it; why is this safe? How will we not
> miss the wakeup that makes condition true?

I was thinking preemption was disabled. But actually yeah, that's a now
stale patch. We should only get rid of the first barrier, that should be
safe.

Thanks,
Davidlohr