Messages in this thread |  | | From | Wanpeng Li <> | Date | Wed, 20 Oct 2021 20:01:28 +0800 | Subject | Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed |
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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] |  |