From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756176Ab2C0TJ7 (ORCPT ); Tue, 27 Mar 2012 15:09:59 -0400 Received: from mail-qa0-f74.google.com ([209.85.216.74]:54429 "EHLO mail-qa0-f74.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754604Ab2C0TJz (ORCPT ); Tue, 27 Mar 2012 15:09:55 -0400 From: Neal Cardwell To: linux-kernel@vger.kernel.org Cc: Neal Cardwell , Ben Segall , Ingo Molnar , Thomas Gleixner Subject: [PATCH] nohz: fix stale jiffies update in tick_nohz_restart() Date: Tue, 27 Mar 2012 15:09:37 -0400 Message-Id: <1332875377-23014-1-git-send-email-ncardwell@google.com> X-Mailer: git-send-email 1.7.7.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Fix tick_nohz_restart() to not use a stale ktime_t "now" value when calling tick_do_update_jiffies64(now). If we reach this point in the loop it means that we crossed a tick boundary since we grabbed the "now" timestamp, so at this point "now" refers to a time in the old jiffy, so using the old value for "now" is incorrect, and is likely to give us a stale jiffies value. In particular, the first time through the loop the tick_do_update_jiffies64(now) call is always a no-op, since the caller, tick_nohz_restart_sched_tick(), will have already called tick_do_update_jiffies64(now) with that "now" value. Note that tick_nohz_stop_sched_tick() already uses the correct approach: when we notice we cross a jiffy boundary, grab a new timestamp with ktime_get(), and *then* update jiffies. Signed-off-by: Neal Cardwell CC: Ingo Molnar CC: Thomas Gleixner --- kernel/time/tick-sched.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 3526038..6a3a5b9 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -534,9 +534,9 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now) hrtimer_get_expires(&ts->sched_timer), 0)) break; } - /* Update jiffies and reread time */ - tick_do_update_jiffies64(now); + /* Reread time and update jiffies */ now = ktime_get(); + tick_do_update_jiffies64(now); } } -- 1.7.7.3