From c4415c993fc2c8bdf2cf0bfbcbb1ac0e0f7a9eaf Mon Sep 17 00:00:00 2001 From: Usama Arif Date: Tue, 7 Jul 2026 08:38:00 -0700 Subject: [PATCH] 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 Signed-off-by: Thomas Gleixner Link: https://patch.msgid.link/20260707153800.542394-1-usama.arif@linux.dev --- kernel/time/hrtimer.c | 18 +++++++++++++----- 1 file 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; + + /* + * 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); } /**