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 ]

UAUsama Arif committed 4 months ago