Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755814AbbFOSee (ORCPT ); Mon, 15 Jun 2015 14:34:34 -0400 Received: from mail-ig0-f182.google.com ([209.85.213.182]:36037 "EHLO mail-ig0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753656AbbFOSe0 (ORCPT ); Mon, 15 Jun 2015 14:34:26 -0400 MIME-Version: 1.0 In-Reply-To: <1433604461.3165.21.camel@stgolabs.net> References: <1432056298-18738-1-git-send-email-dave@stgolabs.net> <1432056298-18738-3-git-send-email-dave@stgolabs.net> <1433604461.3165.21.camel@stgolabs.net> Date: Mon, 15 Jun 2015 11:34:25 -0700 X-Google-Sender-Auth: 6fcXwf7wUOkWKG0csrJ1R0PvWfo Message-ID: Subject: Re: [PATCH 2/4] locking/rtmutex: Use cmp-cmpxchg From: Jason Low To: Davidlohr Bueso , Jason Low Cc: Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Steven Rostedt , Mike Galbraith , "Paul E. McKenney" , Sebastian Andrzej Siewior , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1790 Lines: 44 Hi David, On Sat, Jun 6, 2015 at 8:27 AM, Davidlohr Bueso wrote: > On Fri, 2015-06-05 at 14:38 +0200, Thomas Gleixner wrote: >> On Tue, 19 May 2015, Davidlohr Bueso wrote: >> >> > Avoid unnecessary cmpxchg calls, all of our other locks >> > use it as well. >> > >> > Signed-off-by: Davidlohr Bueso >> > --- >> > kernel/locking/rtmutex.c | 4 +++- >> > 1 file changed, 3 insertions(+), 1 deletion(-) >> > >> > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c >> > index 74188d8..1d5cc9d 100644 >> > --- a/kernel/locking/rtmutex.c >> > +++ b/kernel/locking/rtmutex.c >> > @@ -74,7 +74,9 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock) >> > * set up. >> > */ >> > #ifndef CONFIG_DEBUG_RT_MUTEXES >> > -# define rt_mutex_cmpxchg(l,c,n) (cmpxchg(&l->owner, c, n) == c) >> > +# define rt_mutex_cmpxchg(l,c,n) \ >> > + (l->owner == c && cmpxchg(&l->owner, c, n) == c) >> >> Errm. How does that improve stuff like rt_mutex_lock() ? > > It just avoids a cmpxchg in the fastpath when locked, at the cost of an > extra test when unlocked. CCAS techniques have been proven to boost some > workloads for both rwsems and mutexes. That's all. The CCAS technique was typically used in the slow paths for those other locks, where the chance of the operation returning false is higher. The rt_mutex_cmpxchg is used in places such as rt_mutex fastlocks. We might not want to add extra costs to the fast paths, particularly when the rt_mutex_cmpxchg are marked "likely" in those cases. -- 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/