hrtimer: Don't take cpu_base::lock in hrtimer_get_next_event() when hres_active

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.

The function takes cpu_base->lock, checks hres_active, and returns
KTIME_MAX.

Taking the lock is not required because cpu_base->hres_active is only
written by the local CPU in hrtimer_switch_to_hres() from hard interrupt
context and in 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. No writer can therefore
race with the read, so an unlocked hres_active check is stable and the lock
can be skipped in this case.

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

Move the hres_active check before the lock guard region to address this.

[ tglx: Massaged change log and comments ]

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260707153800.542394-1-usama.arif@linux.dev

1 files changed, 13 insertions, 5 deletions

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 9c320ab7a944..697816d0dc26 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1786,13 +1786,21 @@ 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 cmp_next_hrtimer_event() expects KTIME_MAX.
+	 *
+	 * cpu_base->hres_active is written only by the local CPU in
+	 * hrtimer_switch_to_hres() from hard interrupt context and in
+	 * hrtimers_cpu_starting() during CPU bring-up, and all callers reach
+	 * this with interrupts disabled on the same CPU, so an unlocked read is
+	 * stable without holding the lock.
+	 */
+	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);
 }
 
 /**