Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934152AbZJGPlu (ORCPT ); Wed, 7 Oct 2009 11:41:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933372AbZJGPlu (ORCPT ); Wed, 7 Oct 2009 11:41:50 -0400 Received: from tomts5-srv.bellnexxia.net ([209.226.175.25]:37001 "EHLO tomts5-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933367AbZJGPlt (ORCPT ); Wed, 7 Oct 2009 11:41:49 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AlMFABZOzEpMROOX/2dsb2JhbACBUtVjhCoE Date: Wed, 7 Oct 2009 11:41:11 -0400 From: Mathieu Desnoyers To: Christoph Lameter Cc: Peter Zijlstra , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Pekka Enberg , Tejun Heo , Mel Gorman , mingo@elte.hu Subject: Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable Message-ID: <20091007154111.GA12351@Krystal> References: <20091006233654.815079668@gentwo.org> <20091006233733.153341605@gentwo.org> <20091007025440.GB4664@Krystal> <1254906707.26976.225.camel@twins> <20091007124628.GB27363@Krystal> <20091007150257.GA8508@Krystal> <20091007151940.GA10052@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.27.31-grsec (i686) X-Uptime: 11:31:48 up 50 days, 2:21, 3 users, load average: 0.33, 0.58, 0.41 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1619 Lines: 56 * Christoph Lameter (cl@linux-foundation.org) wrote: > On Wed, 7 Oct 2009, Mathieu Desnoyers wrote: > > > You are already calling the scheduler when ending the _fast_ path. I > > don't see the problem with calling it when you end the slow path > > execution. > > Well yes that gives rise to the thought of using > > preempt_enable_no_sched() > > at the end of the fastpath as well. Constant calls into the scheduler > could be a major performance issue ... but the opposite is a major RT behavior killer. preempt_check_resched is basically: a test TIF_NEED_RESCHED if true, call to preempt_schedule So, it's not a call to the scheduler at each and every preempt_enable. It's _only_ if an interrupt came in during the preempt off section and the scheduler considered that it should re-schedule soon. I really don't see what's bothering you here. Testing a thread flag is incredibly cheap. That's what is typically added to your fast path. So, correct behavior would be: preempt disable() fast path attempt if (fast path already taken) { local_irq_save(); slow path. local_irq_restore(); } preempt_enable() Mathieu > . I dont notice it here since I usually > cannot affort the preempt overhead and build kernels without support for > it. > > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- 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/