Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754563Ab3HPQkt (ORCPT ); Fri, 16 Aug 2013 12:40:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7849 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753223Ab3HPQkq (ORCPT ); Fri, 16 Aug 2013 12:40:46 -0400 Date: Fri, 16 Aug 2013 18:33:55 +0200 From: Oleg Nesterov To: Frederic Weisbecker Cc: LKML , Fernando Luis Vazquez Cao , Tetsuo Handa , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Andrew Morton , Arjan van de Ven Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock Message-ID: <20130816163355.GA766@redhat.com> References: <1376667753-29014-1-git-send-email-fweisbec@gmail.com> <1376667753-29014-3-git-send-email-fweisbec@gmail.com> <20130816160201.GA31682@redhat.com> <20130816163217.GF24210@somewhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130816163217.GF24210@somewhere> 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: 1339 Lines: 42 On 08/16, Frederic Weisbecker wrote: > > On Fri, Aug 16, 2013 at 06:02:01PM +0200, Oleg Nesterov wrote: > > > > Unless I missread this patch, this is still racy a bit. > > > > Suppose it is called on CPU_0 and cpu == 1. Suppose that > > ts->idle_active == T and nr_iowait_cpu(cpu) == 1. > > > > So we return iowait_sleeptime + delta. > > > > Suppose that we call get_cpu_iowait_time_us() again. By this time > > the task which incremented ->nr_iowait can be woken up on another > > CPU, and it can do atomic_dec(rq->nr_iowait). So the next time > > we return iowait_sleeptime, and this is not monotonic again. > > > > No? > > OTOH, io_schedule() does: > > atomic_inc(&rq->nr_iowait); > schedule(); > atomic_dec(&rq->nr_iowait); > > How do we handle that when the task is migrated after it goes to sleep? or even before it goes to sleep. This is what I meant. > I don't either see that iowait tasks can't be migrated. But probably this is fine? This is just the non-precise accounting. But otoh, I agree. The whole idea about per-cpu nr_iowait looks a bit strange. Oleg. -- 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/