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Re: [Xen-devel] Xen spinlock questions

To: Jan Beulich <jbeulich@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] Xen spinlock questions
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 04 Aug 2008 11:24:19 +0100
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On 4/8/08 11:18, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> 1) While the goal of the per-CPU kicker irq appears to be to avoid all CPUs
> waiting for a particular lock to get kicked simultaneously, I think this
> doesn't have the desired effect. This is because Xen doesn't track what
> event channel you poll for (through SCHEDOP_poll), and rather kicks all CPUs
> polling for any event channel.

Yes, this is true. We could easily do something better for VCPUs polling a
single event channel though, but there hasn't been a need up to now. I
suppose it depends how often we have multiple VCPUs stuck waiting for
spinlocks. I can sort out a Xen-side patch if someone wanted to measure the
benefits from more selective wakeup from poll.

 -- Keir

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