[PATCH] hrtimers: skip cpu_base->lock in hrtimer_get_next_event() when hres_active

[Usama Arif]

hrtimer_get_next_event() runs on every tick-stop decision via
get_next_timer_interrupt() -> cmp_next_hrtimer_event().  When high
resolution timers are active it must return KTIME_MAX -- the caller
documents and depends on this.

Yet the current body takes cpu_base->lock, checks hres_active, and
returns KTIME_MAX anyway.  On any modern kernel HRES is on continuously
after boot, so the lock acquisition is pure overhead on every
idle-decision path.

On a 176-thread AMD EPYC 9D64 running a Meta production workload, bucketing
callers of native_queued_spin_lock_slowpath by the _raw_spin_lock*, the
slowpath had 199 samples total in total, of which 54 are attributed to
hrtimer_get_next_event+0x2b (the guard() lock take), i.e. this lock take
accounts for ~27% of irqsave slowpath hits on this specific workload.

cpu_base->hres_active is only written by the local CPU
(hrtimer_switch_to_hres() from hardirq context, hrtimers_cpu_starting()
during bring-up).  All callers of hrtimer_get_next_event() reach it
from the tick-stop / cpuidle paths with interrupts disabled on that
CPU (__get_next_timer_interrupt() itself takes base_local->lock with
raw_spin_lock(), not raw_spin_lock_irqsave()).  No writer can therefore
race with the read, so an unlocked hres_active check is stable and the
lock can be skipped in the common case.

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
 kernel/time/hrtimer.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 313dcea127fe..5be2cb035b86 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1786,13 +1786,19 @@ EXPORT_SYMBOL_GPL(__hrtimer_get_remaining);
 ktime_t hrtimer_get_next_event(void)
 {
 	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
-	ktime_t expires = KTIME_MAX;
 
-	guard(raw_spinlock_irqsave)(&cpu_base->lock);
-	if (!hrtimer_hres_active(cpu_base))
-		expires = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
+	/*
+	 * When HRES is active the caller (cmp_next_hrtimer_event()) expects
+	 * KTIME_MAX.  cpu_base->hres_active is written only by the local CPU
+	 * (hrtimer_switch_to_hres() from hardirq, hrtimers_cpu_starting()
+	 * during bring-up), and all callers reach this with IRQs disabled on
+	 * the same CPU, so an unlocked read is stable across the lock below.
+	 */
+	if (hrtimer_hres_active(cpu_base))
+		return KTIME_MAX;
 
-	return expires;
+	guard(raw_spinlock_irqsave)(&cpu_base->lock);
+	return __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
 }
 
 /**
-- 
2.53.0-Meta