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Message-Id: <1332875377-23014-1-git-send-email-ncardwell@google.com> Date: Tue, 27 Mar 2012 15:09:37 -0400 From: Neal Cardwell <ncardwell@...gle.com> To: linux-kernel@...r.kernel.org Cc: Neal Cardwell <ncardwell@...gle.com>, Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de> Subject: [PATCH] nohz: fix stale jiffies update in tick_nohz_restart() 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 <ncardwell@...gle.com> CC: Ingo Molnar <mingo@...e.hu> CC: Thomas Gleixner <tglx@...utronix.de> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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