[tip: sched/hrtick] hrtimer: Less agressive interrupt 'hang' handling

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/hrtick branch of tip:

Commit-ID:     92f7ee408c23fcc074e4952fb6ffacdf3eb86b56
Gitweb:        https://git.kernel.org/tip/92f7ee408c23fcc074e4952fb6ffacdf3eb86b56
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 10 Mar 2026 20:02:21 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Wed, 11 Mar 2026 21:13:55 +01:00

hrtimer: Less agressive interrupt 'hang' handling

When the hrtimer_interrupt needs to restart more than 3 times and still has
expired timers, the interrupt is considered hung. To give the system a
little time to recover, the hardware timer is programmed a little into the
future.

Prior to commit 288924384856 ("hrtimer: Re-arrange hrtimer_interrupt()"),
this was relative to the amount of time spend serving the interrupt with a
max of 100 msec.

However, in order to simplify, and because this condition 'should' not
happen, the timeout was unconditionally set to 100 msec.

'Obviously' there is a benchmark that hits this hard, by programming a
ton of very short timers :-/

Since reprogramming is decoupled from the interrupt handling, the actual
execution time is lost, however the code does track max_hang_time. Using
that, rather than the 100 ms max restores performance.

  stress-ng --timeout 60 --times --verify --metrics --no-rand-seed --timermix 64

                  bogo ops/s
 288924384856^1: 23715979.93
 288924384856:   11550049.77
 patched:        23361116.78

Additionally, Thomas noted that cpu_base->hang_detected should not be
cleared until the next interrupt, such that __hrtimer_reprogram() won't
undo the extra delay.

Fixes: 288924384856 ("hrtimer: Re-arrange hrtimer_interrupt()")
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Link: https://patch.msgid.link/20260311121500.GF652779@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/oe-lkp/202603102229.74b9dee4-lkp@xxxxxxxxx
---
 kernel/time/hrtimer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index b94bd56..cafd677 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -2031,8 +2031,8 @@ static void hrtimer_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t expires_nex
 		 * Give the system a chance to do something else than looping
 		 * on hrtimer interrupts.
 		 */
-		expires_next = ktime_add_ns(ktime_get(), 100 * NSEC_PER_MSEC);
-		cpu_base->hang_detected = false;
+		expires_next = ktime_add_ns(ktime_get(),
+					    min(100 * NSEC_PER_MSEC, cpu_base->max_hang_time));
 	}
 	hrtimer_rearm_event(expires_next, deferred);
 }
@@ -2121,6 +2121,7 @@ retry:
 	 */
 	now = hrtimer_update_base(cpu_base);
 	expires_next = hrtimer_update_next_event(cpu_base);
+	cpu_base->hang_detected = false;
 	if (expires_next < now) {
 		if (++retries < 3)
 			goto retry;