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

0 files changed, 0 insertions, 0 deletions