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Date:   Wed, 20 Oct 2021 13:17:43 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Wanpeng Li <wanpengli@tencent.com>
Subject: Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is
 needed
Message-ID: <YW/61zpycsD8/z4g@hirez.programming.kicks-ass.net>
References: <20211020110638.797389-1-pbonzini@redhat.com>
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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?