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

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

[Leon Woestenberg]

Hello,

(including linux-rt-users in the CC:, irqthreads are on-topic there)

On Wed, Jul 2, 2008 at 1:02 PM, Andi Kleen <andi@firstfloor.org> wrote:
> Benjamin Herrenschmidt <benh@kernel.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

> 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.

Regards,
-- 
Leon

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

[Andi Kleen]

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@firstfloor.org> wrote:
>> Benjamin Herrenschmidt <benh@kernel.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