linux-kernel - Re: [GIT PULL tip/genirq] Please pull from lost-spurious-irq

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Message-ID: <4C572327.7040801@suse.de>
Date:	Mon, 02 Aug 2010 21:57:27 +0200
From:	Tejun Heo <teheo@...e.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	lkml <linux-kernel@...r.kernel.org>, Jeff Garzik <jeff@...zik.org>,
	Greg KH <gregkh@...e.de>
Subject: Re: [GIT PULL tip/genirq] Please pull from lost-spurious-irq

Hello, Thomas.

On 08/02/2010 08:52 PM, Thomas Gleixner wrote:
>> Ooh, another reason is timer locality.  If timers are shared per desc,
>> they have much higher chance of being on the same processor.  Global
>> timers would be pretty bad in that respect.
> 
> That's irrelevant. If you need to poll an interrupt, then it does not
> matter at all whether you bounce some cache lines or not.
> 
> In fact we have two cases:
> 
>    1) An interrupt needs to be polled all the time. That sucks whether
>       the poll timer bounces a few cache lines or not.
> 
>    2) Polling an irq for some time. Either it works again after a
>       while, so your suckage is restricted to the poll period. If not
>       see #1

Hmm... for spurious and watch the above are true and if it were the
above two it would definitely make more sense to use per-purpose
global timers.  The problem is w/ expect tho.  It's supposed to be
used with normal hot paths, so expect/unexpect operations better be
low overhead and local.  I'll talk more about it in the other reply.

> And it's even less of an issue as the main users of this misfeature
> are laptops and desktop machines, where locality is not really that
> important. If an enterprise admin decides to ignore the fact that the
> hardware is flaky, then he does not care about the cache line bounces
> either.

These problems do happen on intel chipset machines and is something
which can be worked around with some effort.  Eh, let's talk on the
other reply.

Thanks.

-- 
tejun
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