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Subjectkernel/timer: avoid spurious ksoftirqd wakeups

It is only necessary to raise timer softirq
in case there are active timers or irq work
to do.

Limit the ksoftirqd wakeup to those cases.

Fixes a latency spike with isolated CPUs and
nohz full mode.

Reported-and-tested-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 8c5a197..0c065f9 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -192,7 +192,7 @@ extern void set_timer_slack(struct timer_list *time, int slack_hz);
* locks the timer base and does the comparison against the given
* jiffie.
*/
-extern unsigned long get_next_timer_interrupt(unsigned long now);
+extern unsigned long get_next_timer_interrupt(unsigned long now, bool *raise_softirq);

/*
* Timer-statistics info:
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a4c4eda..615e276 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -568,6 +568,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
unsigned long rcu_delta_jiffies;
struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
u64 time_delta;
+ bool raise_softirq;

time_delta = timekeeping_max_deferment();

@@ -582,9 +583,11 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
arch_needs_cpu() || irq_work_needs_cpu()) {
next_jiffies = last_jiffies + 1;
delta_jiffies = 1;
+ raise_softirq = true;
} else {
/* Get the next timer wheel timer */
- next_jiffies = get_next_timer_interrupt(last_jiffies);
+ next_jiffies = get_next_timer_interrupt(last_jiffies,
+ &raise_softirq);
delta_jiffies = next_jiffies - last_jiffies;
if (rcu_delta_jiffies < delta_jiffies) {
next_jiffies = last_jiffies + rcu_delta_jiffies;
@@ -703,7 +706,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
*/
tick_do_update_jiffies64(ktime_get());
}
- raise_softirq_irqoff(TIMER_SOFTIRQ);
+ if (raise_softirq)
+ raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
ts->next_jiffies = next_jiffies;
ts->last_jiffies = last_jiffies;
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d3f5c5..771f811 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1343,7 +1343,7 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
* get_next_timer_interrupt - return the jiffy of the next pending timer
* @now: current time (in jiffies)
*/
-unsigned long get_next_timer_interrupt(unsigned long now)
+unsigned long get_next_timer_interrupt(unsigned long now, bool *raise_softirq)
{
struct tvec_base *base = __this_cpu_read(tvec_bases);
unsigned long expires = now + NEXT_TIMER_MAX_DELTA;
@@ -1357,6 +1357,7 @@ unsigned long get_next_timer_interrupt(unsigned long now)

spin_lock(&base->lock);
if (base->active_timers) {
+ *raise_softirq = true;
if (time_before_eq(base->next_timer, base->timer_jiffies))
base->next_timer = __next_timer_interrupt(base);
expires = base->next_timer;



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