linux-kernel - Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

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Message-ID: <54B6B3F3.2000908@redhat.com>
Date:	Wed, 14 Jan 2015 13:22:43 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
CC:	Luiz Capitulino <lcapitulino@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On 01/14/2015 12:12 PM, Marcelo Tosatti wrote:
> The problem:
> 
> On -RT, an emulated LAPIC timer instances has the following path:
> 
> 1) hard interrupt
> 2) ksoftirqd is scheduled
> 3) ksoftirqd wakes up vcpu thread
> 4) vcpu thread is scheduled
> 
> This extra context switch introduces unnecessary latency in the 
> LAPIC path for a KVM guest.

> This patch reduces the average latency in my tests from 14us to 11us.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>

Acked-by: Rik van Riel <riel@...hat.com>

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