linux-kernel - Re: [PATCH RFC 3/3] x86: use mwait for trigger API

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Message-ID: <20080816153109.3469878c@infradead.org>
Date:	Sat, 16 Aug 2008 15:31:09 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH RFC 3/3] x86: use mwait for trigger API

On Sat, 16 Aug 2008 14:50:34 -0700
Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Arjan van de Ven wrote:
> > monitor/mwait is rather really expensive.. are we really sure we
> > want to use this?
> > (from an Intel cpu perspective the answer is very likely no; but I
> > don't know what AMD does here)
> >   
> 
> The intended use is when you're going to be waiting for a while (on
> the order of microseconds or more). 

well mwait really is not cheap, I'd not be surprised if it's in that
same order.

> In the Xen case, I use this to
> block the vcpu if we pass a few iterations without the condition
> being true. While the mwait patch doesn't do this at present, it
> could.

that's another hard one.. passing C-state hints into mwait needs ACPI
help; the BIOS tells us which mwait values are legal/valid at any point
in time.. but this gets tricky to put into these spinpletions.


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