linux-kernel - Re: [PATCH 4/5] kvm: Replace vcpu->swait with rcuwait

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Date:   Wed, 22 Apr 2020 11:14:28 +0100
From:   Marc Zyngier <maz@...terjones.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     tglx@...utronix.de, pbonzini@...hat.com, kvm@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>, peterz@...radead.org,
        torvalds@...ux-foundation.org, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        linux-mips@...r.kernel.org, Paul Mackerras <paulus@...abs.org>,
        joel@...lfernandes.org, will@...nel.org,
        kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH 4/5] kvm: Replace vcpu->swait with rcuwait

On 2020-04-22 05:07, Davidlohr Bueso wrote:
> The use of any sort of waitqueue (simple or regular) for
> wait/waking vcpus has always been an overkill and semantically
> wrong. Because this is per-vcpu (which is blocked) there is
> only ever a single waiting vcpu, thus no need for any sort of
> queue.
> 
> As such, make use of the rcuwait primitive, with the following
> considerations:
> 
>   - rcuwait already provides the proper barriers that serialize
>   concurrent waiter and waker.
> 
>   - Task wakeup is done in rcu read critical region, with a
>   stable task pointer.
> 
>   - Because there is no concurrency among waiters, we need
>   not worry about rcuwait_wait_event() calls corrupting
>   the wait->task. As a consequence, this saves the locking
>   done in swait when modifying the queue. This also applies
>   to per-vcore wait for powerpc kvm-hv.
> 
> The x86 tscdeadline_latency test mentioned in 8577370fb0cb
> ("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg,
> latency is reduced by around 15-20% with this change.
> 
> Cc: Paul Mackerras <paulus@...abs.org>
> Cc: kvmarm@...ts.cs.columbia.edu
> Cc: linux-mips@...r.kernel.org
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>

Reviewed-by: Marc Zyngier <maz@...nel.org>

Thanks,

         M.
-- 
Who you jivin' with that Cosmik Debris?