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

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Message-ID: <5474D121.4070905@redhat.com>
Date:	Tue, 25 Nov 2014 13:57:37 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
CC:	Luiz Capitulino <lcapitulino@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On 11/25/2014 12:21 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.
> 
> The solution:
> 
> Allow waking up vcpu thread from hardirq context,
> thus avoiding the need for ksoftirqd to be scheduled.
> 
> Normal waitqueues make use of spinlocks, which on -RT 
> are sleepable locks. Therefore, waking up a waitqueue 
> waiter involves locking a sleeping lock, which 
> is not allowed from hard interrupt context.
> 

What are the consequences for the realtime kernel of
making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so
long we need to care about it being non-preemptible?


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