Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752562AbaFLUzE (ORCPT ); Thu, 12 Jun 2014 16:55:04 -0400 Received: from g6t1525.atlanta.hp.com ([15.193.200.68]:1103 "EHLO g6t1525.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750751AbaFLUzA (ORCPT ); Thu, 12 Jun 2014 16:55:00 -0400 Message-ID: <539A139C.50400@hp.com> Date: Thu, 12 Jun 2014 16:54:52 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, Paolo Bonzini , Konrad Rzeszutek Wilk , Boris Ostrovsky , "Paul E. McKenney" , Rik van Riel , Linus Torvalds , Raghavendra K T , David Vrabel , Oleg Nesterov , Gleb Natapov , Scott J Norton , Chegu Vinod Subject: Re: [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit path References: <1401464642-33890-1-git-send-email-Waiman.Long@hp.com> <1401464642-33890-7-git-send-email-Waiman.Long@hp.com> <20140611102606.GK3213@twins.programming.kicks-ass.net> <5398C894.6040808@hp.com> <20140612060032.GQ6758@twins.programming.kicks-ass.net> In-Reply-To: <20140612060032.GQ6758@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/12/2014 02:00 AM, Peter Zijlstra wrote: > On Wed, Jun 11, 2014 at 05:22:28PM -0400, Long, Wai Man wrote: > >>>> @@ -233,11 +233,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) >>>> */ >>>> for (;;) { >>>> /* >>>> - * If we observe any contention; queue. >>>> + * If we observe that the queue is not empty or both >>>> + * the pending and lock bits are set, queue >>>> */ >>>> - if (val& ~_Q_LOCKED_MASK) >>>> + if ((val& _Q_TAIL_MASK) || >>>> + (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL))) >>>> goto queue; >>>> + if (val == _Q_PENDING_VAL) { >>>> + /* >>>> + * Pending bit is set, but not the lock bit. >>>> + * Assuming that the pending bit holder is going to >>>> + * set the lock bit and clear the pending bit soon, >>>> + * it is better to wait than to exit at this point. >>>> + */ >>>> + cpu_relax(); >>>> + val = atomic_read(&lock->val); >>>> + continue; >>>> + } >>>> + >>>> new = _Q_LOCKED_VAL; >>>> if (val == new) >>>> new |= _Q_PENDING_VAL; >>> Wouldn't something like: >>> >>> while (atomic_read(&lock->val) == _Q_PENDING_VAL) >>> cpu_relax(); >>> >>> before the cmpxchg loop have gotten you all this? >> That is not exactly the same. The loop will exit if other bits are set or the pending >> bit cleared. In the case, we will need to do the same check at the beginning of the >> for loop in order to avoid doing an extra cmpxchg that is not necessary. > If other bits get set we should stop poking at the pending bit and get > queued. The only transition we want to wait for is: 0,1,0 -> 0,0,1. > > What extra unneeded cmpxchg() is there? If we have two cpus waiting in > this loop for the pending bit to go away then both will attempt to grab > the now free pending bit, one will loose and get queued? > > There's no avoiding that contention. If two tasks see the pending bit goes away and try to grab it with cmpxchg, there is no way we can avoid the contention. However, if some how the pending bit holder get the lock and another task set the pending bit before the current task, the spinlock value will become _Q_PENDING_VAL|_Q_LOCKED_VAL. The while loop will end and the code will blindly try to do a cmpxchg unless we check for this case before hand. This is what my code does by going back to the beginning of the for loop. -Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/