[tip: timers/core] tick: Shut down low-res tick from dying CPU

tip-bot2 for Frederic Weisbecker
2024-02-26T22:24:31Z

The following commit has been merged into the timers/core branch of tip:

Commit-ID: 3f69d04e146c6e14ccdd4e7b37d93f789229202a
Gitweb: https://git.kernel.org/tip/3f69d04e146c6e14ccdd4e7b37d93f789229202a
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Sun, 25 Feb 2024 23:55:06 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 26 Feb 2024 11:37:32 +01:00

tick: Shut down low-res tick from dying CPU

The timekeeping duty is handed over from the outgoing CPU within stop
machine. This works well if CONFIG_NO_HZ_COMMON=n or the tick is in
high-res mode. However in low-res dynticks mode, the tick isn't
cancelled until the clockevent is shut down, which can happen later. The
tick may therefore fire again once IRQs are re-enabled on stop machine
and until IRQs are disabled for good upon the last call to idle.

That's so many opportunities for a timekeeper to go idle and the
outgoing CPU to take over that duty. This is why
tick_nohz_idle_stop_tick() is called one last time on idle if the CPU
is seen offline: so that the timekeeping duty is handed over again in
case the CPU has re-taken the duty.

This means there are two timekeeping handovers on CPU down hotplug with
different undocumented constraints and purposes:

1) A handover on stop machine for !dynticks || highres. All online CPUs
are guaranteed to be non-idle and the timekeeping duty can be safely
handed-over. The hrtimer tick is cancelled so it is guaranteed that in
dynticks mode the outgoing CPU won't take again the duty.

2) A handover on last idle call for dynticks && lowres. Setting the
duty to TICK_DO_TIMER_NONE makes sure that a CPU will take over the
timekeeping.

Prepare for consolidating the handover to a single place (the first one)
with shutting down the low-res tick as well from
tick_cancel_sched_timer() as well. This will simplify the handover and
unify the tick cancellation between high-res and low-res.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240225225508.11587-15-frederic@kernel.org

---
kernel/time/tick-common.c | 3 ++-
kernel/time/tick-sched.c | 32 +++++++++++++++++++++++++-------
kernel/time/tick-sched.h | 4 ++--
3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 5224140..9cd09ee 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -410,7 +410,8 @@ int tick_cpu_dying(unsigned int dying_cpu)
if (tick_do_timer_cpu == dying_cpu)
tick_do_timer_cpu = cpumask_first(cpu_online_mask);

- tick_cancel_sched_timer(dying_cpu);
+ /* Make sure the CPU won't try to retake the timekeeping duty */
+ tick_sched_timer_dying(dying_cpu);

/* Remove CPU from timer broadcasting */
tick_offline_cpu(dying_cpu);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index dcb9f03..89d16b8 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -308,6 +308,14 @@ static enum hrtimer_restart tick_nohz_handler(struct hrtimer *timer)
return HRTIMER_RESTART;
}

+static void tick_sched_timer_cancel(struct tick_sched *ts)
+{
+ if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
+ hrtimer_cancel(&ts->sched_timer);
+ else if (tick_sched_flag_test(ts, TS_FLAG_NOHZ))
+ tick_program_event(KTIME_MAX, 1);
+}
+
#ifdef CONFIG_NO_HZ_FULL
cpumask_var_t tick_nohz_full_mask;
EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
@@ -1040,10 +1048,7 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
* the tick timer.
*/
if (unlikely(expires == KTIME_MAX)) {
- if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
- hrtimer_cancel(&ts->sched_timer);
- else
- tick_program_event(KTIME_MAX, 1);
+ tick_sched_timer_cancel(ts);
return;
}

@@ -1598,14 +1603,27 @@ void tick_setup_sched_timer(bool hrtimer)
tick_nohz_activate(ts);
}

-void tick_cancel_sched_timer(int cpu)
+/*
+ * Shut down the tick and make sure the CPU won't try to retake the timekeeping
+ * duty before disabling IRQs in idle for the last time.
+ */
+void tick_sched_timer_dying(int cpu)
{
+ struct tick_device *td = &per_cpu(tick_cpu_device, cpu);
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ struct clock_event_device *dev = td->evtdev;
ktime_t idle_sleeptime, iowait_sleeptime;
unsigned long idle_calls, idle_sleeps;

- if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
- hrtimer_cancel(&ts->sched_timer);
+ /* This must happen before hrtimers are migrated! */
+ tick_sched_timer_cancel(ts);
+
+ /*
+ * If the clockevents doesn't support CLOCK_EVT_STATE_ONESHOT_STOPPED,
+ * make sure not to call low-res tick handler.
+ */
+ if (tick_sched_flag_test(ts, TS_FLAG_NOHZ))
+ dev->event_handler = clockevents_handle_noop;

idle_sleeptime = ts->idle_sleeptime;
iowait_sleeptime = ts->iowait_sleeptime;
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index bbe72a0..58d8d1c 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -106,9 +106,9 @@ extern struct tick_sched *tick_get_tick_sched(int cpu);

extern void tick_setup_sched_timer(bool hrtimer);
#if defined CONFIG_NO_HZ_COMMON || defined CONFIG_HIGH_RES_TIMERS
-extern void tick_cancel_sched_timer(int cpu);
+extern void tick_sched_timer_dying(int cpu);
#else
-static inline void tick_cancel_sched_timer(int cpu) { }
+static inline void tick_sched_timer_dying(int cpu) { }
#endif

#ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST