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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Rik van Riel <riel@redhat.com>,
	Luiz Capitulino <lcapitulino@redhat.com>,
	linux-rt-users@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v5)
Date: Thu, 9 Apr 2015 15:18:21 +0200	[thread overview]
Message-ID: <20150409131821.GA1160@linutronix.de> (raw)
In-Reply-To: <20150408233323.842124241@redhat.com>

* Marcelo Tosatti | 2015-04-08 20:33:23 [-0300]:

>Sebastian,
Hi Marcelo,

>rebased against v3.18.7-rt2 as requested.
>
>The problem:
>
>On -RT, an emulated LAPIC timer instance 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.

Applied, thanks.

Sebastian

      parent reply	other threads:[~2015-04-09 13:18 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-08 23:33 [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v5) Marcelo Tosatti
2015-04-08 23:33 ` [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq Marcelo Tosatti
2015-04-08 23:33 ` [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe Marcelo Tosatti
2015-04-09 13:18 ` Sebastian Andrzej Siewior [this message]

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