Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756701AbbDOUFI (ORCPT ); Wed, 15 Apr 2015 16:05:08 -0400 Received: from g1t5425.austin.hp.com ([15.216.225.55]:36622 "EHLO g1t5425.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756190AbbDOUEv (ORCPT ); Wed, 15 Apr 2015 16:04:51 -0400 Message-ID: <1429128287.7039.145.camel@j-VirtualBox> Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer to improve scalability From: Jason Low To: Peter Zijlstra Cc: Frederic Weisbecker , Linus Torvalds , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, "Paul E. McKenney" , Andrew Morton , Oleg Nesterov , Mike Galbraith , Mel Gorman , Steven Rostedt , Preeti U Murthy , hideaki.kimura@hp.com, Aswin Chandramouleeswaran , Scott J Norton , jason.low2@hp.com Date: Wed, 15 Apr 2015 13:04:47 -0700 In-Reply-To: <20150415133211.GP23123@twins.programming.kicks-ass.net> References: <1429052986-9420-1-git-send-email-jason.low2@hp.com> <1429052986-9420-3-git-send-email-jason.low2@hp.com> <20150415132534.GB4489@lerouge> <20150415133211.GP23123@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2296 Lines: 52 On Wed, 2015-04-15 at 15:32 +0200, Peter Zijlstra wrote: > On Wed, Apr 15, 2015 at 03:25:36PM +0200, Frederic Weisbecker wrote: > > On Tue, Apr 14, 2015 at 04:09:45PM -0700, Jason Low wrote: > > > void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times) > > > { > > > struct thread_group_cputimer *cputimer = &tsk->signal->cputimer; > > > struct task_cputime sum; > > > > > > if (!cputimer->running) { > > > /* > > > * The POSIX timer interface allows for absolute time expiry > > > * values through the TIMER_ABSTIME flag, therefore we have > > > + * to synchronize the timer to the clock every time we start it. > > > */ > > > thread_group_cputime(tsk, &sum); > > > + update_gt_cputime(cputimer, &sum); > > > + /* Start 'running' after update_gt_cputime() */ > > > + smp_store_release(&cputimer->running, 1); > > > > This barrier should be mirrored somewhere but I can't see where in this patch. > > Maybe in another one in the series. Or maybe there is already a barrier in the > > existing code that I'm missing. I would expect to see it in account_group_*_time(). > > In any case, there should be comment about what it mirrors. > > I think it should be in cputimer_running(), which should use > smp_load_acquire() to read cputimer->running. > > That way you guarantee that everything observing 'running' will indeed > observe the initialized state. So I intended the smp_store_release() here to be mainly for documentation purposes, to say that we would like to set running after the update. With patch 3/3, even if running happens to get set earlier, the worst case scenario is that update_gt_cputime may have to do go through some retry logic. This isn't much of a performance issue in practice (especially compared to adding smp_load_acquire() to hot paths), since we only enter this path when we need to enable the timers. In that case, I'm wondering if should just convert this back to WRITE_ONCE(cputimer->running, 1) and avoid adding barriers to the hot paths? Thanks, Jason -- 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/