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Subject[PATCH 1/4] tick/sched: Forward timer even in nohz mode
Date
Currently when exiting nohz, the expiry will be forwarded as if we
had just run the timer. If we re-enter nohz before this new expiry,
and exit after, this forwarding will happen again. If this load pattern
recurs the tick can be indefinitely postponed.

To avoid this, use last_tick as-is rather than calling hrtimer_forward().
However, in some cases the tick *will* have just run (despite being
"stopped"), and leading to double timer execution.

To avoid that, forward the timer after every tick (regardless of nohz
status) and keep last_tick up-to-date. During nohz, last_tick will
reflect what the expiry would have been if not in nohz mode.

Signed-off-by: Scott Wood <swood@redhat.com>
---
kernel/time/tick-sched.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 8b192e67aabc..8936b604dd6c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -642,9 +642,6 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
hrtimer_cancel(&ts->sched_timer);
hrtimer_set_expires(&ts->sched_timer, ts->last_tick);

- /* Forward the time to expire in the future */
- hrtimer_forward(&ts->sched_timer, now, tick_period);
-
if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
hrtimer_start_expires(&ts->sched_timer,
HRTIMER_MODE_ABS_PINNED_HARD);
@@ -1207,12 +1204,13 @@ static void tick_nohz_handler(struct clock_event_device *dev)

tick_sched_do_timer(ts, now);
tick_sched_handle(ts, regs);
+ hrtimer_forward(&ts->sched_timer, now, tick_period);
+ ts->last_tick = hrtimer_get_expires(&ts->sched_timer);

/* No need to reprogram if we are running tickless */
if (unlikely(ts->tick_stopped))
return;

- hrtimer_forward(&ts->sched_timer, now, tick_period);
tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
}

@@ -1311,12 +1309,13 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
else
ts->next_tick = 0;

+ hrtimer_forward(timer, now, tick_period);
+ ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
+
/* No need to reprogram if we are in idle or full dynticks mode */
if (unlikely(ts->tick_stopped))
return HRTIMER_NORESTART;

- hrtimer_forward(timer, now, tick_period);
-
return HRTIMER_RESTART;
}

--
1.8.3.1
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