From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760090AbbA0V5b (ORCPT ); Tue, 27 Jan 2015 16:57:31 -0500 Received: from smtp2.provo.novell.com ([137.65.250.81]:43653 "EHLO smtp2.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755834AbbA0V5a (ORCPT ); Tue, 27 Jan 2015 16:57:30 -0500 Message-ID: <1422395843.4604.9.camel@stgolabs.net> Subject: Re: [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping From: Davidlohr Bueso To: Peter Zijlstra Cc: Ingo Molnar , "Paul E. McKenney" , Jason Low , Michel Lespinasse , Tim Chen , linux-kernel@vger.kernel.org Date: Tue, 27 Jan 2015 13:57:23 -0800 In-Reply-To: <20150127173442.GK21418@twins.programming.kicks-ass.net> References: <1422257769-14083-1-git-send-email-dave@stgolabs.net> <1422257769-14083-6-git-send-email-dave@stgolabs.net> <20150127173442.GK21418@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.7 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > --- > > 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