On 2020-12-07 08:06:48 [-0800], Paul E. McKenney wrote: > > Yes, but it triggers frequently. Like `rcuc' is somehow is aligned with > > the timeout. > > Given that a lot of RCU processing is event-driven based on timers, > and given that the scheduling-clock interrupts are synchronized for > energy-efficiency reasons on many configs, maybe this alignment is > expected behavior?
No, it is the fact that rcu_preempt has a higher priority than ksoftirqd. So immediately after the wakeup (of rcu_preempt) there is a context switch and expire_timers() has this: | raw_spin_unlock_irq(&base->lock); | call_timer_fn(timer, fn, baseclk); | raw_spin_lock_irq(&base->lock); | base->running_timer = NULL; | timer_sync_wait_running(base); So ->running_timer isn't reset and try_to_del_timer_sync() (that del_timer_sync() from schedule_timeout()) returns -1 and then the corner case is handled where `expiry_lock' is acquired. So everything goes as expected. > Thanx, Paul Sebastian