Messages in this thread | | | Date | Wed, 20 Oct 2021 13:52:54 +0200 | Subject | Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed | From | Paolo Bonzini <> |
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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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