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

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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
This commit is contained in:
Usama Arif
2026-07-07 23:26:58 +02:00
committed by Thomas Gleixner
parent 1d28a67d49
commit c4415c993f
1 changed files with 13 additions and 5 deletions
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kernel/time/hrtimer.c
+13 -5
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@@ -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;
/*
* 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;
guard(raw_spinlock_irqsave)(&cpu_base->lock);
if (!hrtimer_hres_active(cpu_base))
expires = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
return expires;
return __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
}
/**