Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed

[Peter Zijlstra <peterz@infradead.org>]

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?