[PATCH v2] nohz: fix setting next jiffies while switch to low-res nohz mode

[PATCH v2] nohz: fix setting next jiffies while switch to low-res nohz mode

[Wanpeng Li]

After commit 0ff53d096422 ("tick: sched: Force tick interrupt and get rid
of softirq magic"), next expire time is overwritten by last_jiffies_update
while switch to low-res nohz mode which leads to wrong next jiffies. 
This patch fix it by adjusting the order of forward/set in order to program 
ce device the right expire time in the future.

Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * correct patch description

 kernel/time/tick-sched.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9d7a053..a126d16 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -993,9 +993,9 @@ static void tick_nohz_switch_to_nohz(void)
 	/* Get the next period */
 	next = tick_init_jiffy_update();
 
-	hrtimer_forward_now(&ts->sched_timer, tick_period);
 	hrtimer_set_expires(&ts->sched_timer, next);
-	tick_program_event(next, 1);
+	hrtimer_forward_now(&ts->sched_timer, tick_period);
+	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 	tick_nohz_activate(ts, NOHZ_MODE_LOWRES);
 }
 
-- 
1.9.1

[tip:timers/urgent] tick/nohz: Set the correct expiry when switching to nohz/lowres mode

[tip-bot for Wanpeng Li]

Commit-ID:  1ca8ec532fc2d986f1f4a319857bb18e0c9739b4
Gitweb:     http://git.kernel.org/tip/1ca8ec532fc2d986f1f4a319857bb18e0c9739b4
Author:     Wanpeng Li <kernellwp@gmail.com>
AuthorDate: Wed, 27 Jan 2016 19:26:07 +0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 27 Jan 2016 12:45:57 +0100

tick/nohz: Set the correct expiry when switching to nohz/lowres mode

commit 0ff53d096422 sets the next tick interrupt to the last jiffies update,
i.e. in the past, because the forward operation is invoked before the set
operation. There is no resulting damage (yet), but we get an extra pointless
tick interrupt.

Revert the order so we get the next tick interrupt in the future.

Fixes: commit 0ff53d096422 "tick: sched: Force tick interrupt and get rid of softirq magic"
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1453893967-3458-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/tick-sched.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index cbe5d8d..de2d9fe 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -995,9 +995,9 @@ static void tick_nohz_switch_to_nohz(void)
 	/* Get the next period */
 	next = tick_init_jiffy_update();
 
-	hrtimer_forward_now(&ts->sched_timer, tick_period);
 	hrtimer_set_expires(&ts->sched_timer, next);
-	tick_program_event(next, 1);
+	hrtimer_forward_now(&ts->sched_timer, tick_period);
+	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 	tick_nohz_activate(ts, NOHZ_MODE_LOWRES);
 }