linux-kernel - Re: [PATCH 09/12] irq: implement IRQ expecting

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Message-ID: <4C1A5197.8060409@kernel.org>
Date:	Thu, 17 Jun 2010 18:47:19 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...e.hu,
	bphilips@...e.de, yinghai@...nel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jeff@...zik.org, linux-ide@...r.kernel.org,
	stern@...land.harvard.edu, gregkh@...e.de, khali@...ux-fr.org
Subject: Re: [PATCH 09/12] irq: implement IRQ expecting

On 06/17/2010 06:02 PM, Arjan van de Ven wrote:
> On Thu, 17 Jun 2010 17:54:48 +0200
> Tejun Heo <tj@...nel.org> wrote:
> 
>> Crazy devices too but I think they would
>> fall in a single tick any way. 
> 
> not sure what ticks have to do with anything but ok ;)

Eh... right, I was thinking about something else.  IRQ expect code
originally had a tick based duration estimator to determine poll
interval which I ripped out later for simpler stepped adjustments.
c-state would need higher frequency timing measurements than jiffies.

>> At any rate, let's say I have those
>> numbers, how would I feed it into c-state selection?
> 
> if we have this, we need to put a bit of glue in the backend that
> tracks (per cpu I suppose) the shortest expected interrupt, which
> the C state code then queries.
> (and in that regard, it does not matter if shortest expected is
> computed via heuristic on a per irq basis, or passed in).
> 
> mapping an irq to a cpu is not a 100% science (since interrupts can
> move in theory), but just assuming that the irq will happen on the same
> CPU it happened last time is more than good enough.

Hmmm... the thing is that there will be many cases which won't fit
irq_expect() model (why irq_watch() exists in the first place) and for
the time being libata is the only one providing that data.  Would the
data still be useful to determine which c-state to use?

Thanks.

-- 
tejun
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