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Date: Tue, 27 Jan 2015 18:34:42 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Davidlohr Bueso <dave@...olabs.net> Cc: Ingo Molnar <mingo@...nel.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Jason Low <jason.low2@...com>, Michel Lespinasse <walken@...gle.com>, Tim Chen <tim.c.chen@...ux.intel.com>, linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de> Subject: Re: [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping 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@...e.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? > > raw_spin_lock_irq(&sem->wait_lock); > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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