On Wed, Oct 20, 2021 at 07:06:38AM -0400, Paolo Bonzini wrote: > In some cases, rcuwait_wake_up can be called even if the actual likelihood > of a wakeup is very low. If CONFIG_PREEMPT_RCU is active, the resulting > rcu_read_lock/rcu_read_unlock pair can be relatively expensive, and in > fact it is unnecessary when there is no w->task to keep alive: the > memory barrier before the read is what matters in order to avoid missed > wakeups. > > Therefore, do an early check of w->task right after the barrier, and skip > rcu_read_lock/rcu_read_unlock unless there is someone waiting for a wakeup. > > Running kvm-unit-test/vmexit.flat with APICv disabled, most interrupt > injection tests (tscdeadline*, self_ipi*, x2apic_self_ipi*) improve > by around 600 cpu cycles. *how* ?!? AFAICT, rcu_read_lock() for PREEMPT_RCU is: WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1); barrier(); Paul?
