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

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