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

[Paolo Bonzini]

On 20/10/21 13:17, Peter Zijlstra wrote:
> 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?

Wanpeng, can you share your full .config?

Paolo