Messages in this thread Patch in this message | | | Date | Thu, 19 Apr 2012 14:03:41 -0700 | From | Greg KH <> | Subject | [ 38/75] nohz: Fix stale jiffies update in tick_nohz_restart() |
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3.3-stable review patch. If anyone has any objections, please let me know.
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From: Neal Cardwell <ncardwell@google.com>
commit 6f103929f8979d2638e58d7f7fda0beefcb8ee7e upstream.
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@google.com> Cc: Ben Segall <bsegall@google.com> Cc: Ingo Molnar <mingo@elte.hu> Link: http://lkml.kernel.org/r/1332875377-23014-1-git-send-email-ncardwell@google.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--- kernel/time/tick-sched.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -538,9 +538,9 @@ static void tick_nohz_restart(struct tic 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); } }
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