linux-kernel - Re: [Ksummit-2008-discuss] Delayed interrupt work, thread pools

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Message-ID: <486B658A.1040203@firstfloor.org>
Date:	Wed, 02 Jul 2008 13:24:58 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Leon Woestenberg <leon.woestenberg@...il.com>
CC:	benh@...nel.crashing.org, Arjan van de Ven <arjan@...ux.intel.com>,
	ksummit-2008-discuss@...ts.linux-foundation.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Jeremy Kerr <jk@...abs.org>,
	RT <linux-rt-users@...r.kernel.org>
Subject: Re: [Ksummit-2008-discuss] Delayed interrupt work, thread pools

Leon Woestenberg wrote:
> Hello,
> 
> (including linux-rt-users in the CC:, irqthreads are on-topic there)

Actually it's probably not interesting for this case.

> 
> On Wed, Jul 2, 2008 at 1:02 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> Benjamin Herrenschmidt <benh@...nel.crashing.org> writes:
>>
>>>> how much of this would be obsoleted if we had irqthreads ?
>>> I'm not sure irqthreads is what I want...
>>>
>> I also think interrupts threads are a bad idea in many cases because
>> their whole "advantage" over classical interrupts is that they can
>> block. Now blocking can be usually take a unbounded potentially long
>> time.
>>
>> What do you do when there are more interrupts in that unbounded time?
>>
> If by irqthreads the -rt implementation is meant, isn't this what happens:
> 
> irq kernel handler masks the source interrupt
> irq handler awakes the matching irqthread (they always are present)
> irqthread is scheduled, does work and returns
> irq kernel unmasks the source interrupt

I described this case. If the interrupt handler blocks for a long
time (as Ben asked for) then the interrupts will not be handled
for a long time. Probably not what you want.

BTW this was not a criticsm of rt linux (in whose context
irqthreads make sense as I explained) , just an explanation why they
imho don't make sense (IMHO) in a non hard rt interruptible kernel
and especially not to solve Ben's issue.

> 
>> Create more interrupt threads?  At some point you'll have hundreds
>> of threads doing nothing when you're unlucky.
>>
> Each irqthread handles one irq.
> So now new irq thread would spawn for any interrupt.

It was a general description of all possible irqthreads.

-Andi
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