Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752578Ab3HPQJO (ORCPT ); Fri, 16 Aug 2013 12:09:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44508 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751634Ab3HPQJJ (ORCPT ); Fri, 16 Aug 2013 12:09:09 -0400 Date: Fri, 16 Aug 2013 18:02:01 +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: <20130816160201.GA31682@redhat.com> References: <1376667753-29014-1-git-send-email-fweisbec@gmail.com> <1376667753-29014-3-git-send-email-fweisbec@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1376667753-29014-3-git-send-email-fweisbec@gmail.com> 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: 1643 Lines: 50 Thanks Frederic! I'll try to read this series carefully later. Not that I think I can help, you certainly understand this much better. Just one question below, On 08/16, Frederic Weisbecker wrote: > > @@ -499,12 +509,15 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time) > if (last_update_time) > *last_update_time = ktime_to_us(now); > > - if (ts->idle_active && nr_iowait_cpu(cpu) > 0) { > - ktime_t delta = ktime_sub(now, ts->idle_entrytime); > - iowait = ktime_add(ts->iowait_sleeptime, delta); > - } else { > - iowait = ts->iowait_sleeptime; > - } > + do { > + seq = read_seqcount_begin(&ts->sleeptime_seq); > + if (ts->idle_active && nr_iowait_cpu(cpu) > 0) { > + ktime_t delta = ktime_sub(now, ts->idle_entrytime); > + iowait = ktime_add(ts->iowait_sleeptime, delta); > + } else { > + iowait = ts->iowait_sleeptime; > + } > + } while (read_seqcount_retry(&ts->sleeptime_seq, seq)); 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? 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/