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

From: Tejun Heo
Date: Mon Aug 02 2010 - 17:08:40 EST

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.



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