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

[Wanpeng Li]

On Wed, 20 Oct 2021 at 19:53, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> 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?

Yes, in the attachment.

    Wanpeng

Attachment: config
Description: Binary data