lkml.org 
[lkml]   [2021]   [Oct]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed
On Wed, 20 Oct 2021 at 19:53, Paolo Bonzini <pbonzini@redhat.com> 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
[unhandled content-type:application/octet-stream]
\
 
 \ /
  Last update: 2021-10-20 14:01    [W:1.513 / U:0.092 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site