linux-kernel - Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF

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Date:	Thu, 20 May 2010 07:39:27 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jan-Bernd Themann <THEMANN@...ibm.com>, michael@...erman.id.au,
	Brian King <brking@...ux.vnet.ibm.com>,
	Doug Maxey <doug.maxey@...ibm.com>, dvhltc@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	niv@...ux.vnet.ibm.com, Will Schmidt <will_schmidt@...t.ibm.com>
Subject: Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)

On 05/20/2010 01:14 AM, Thomas Gleixner wrote:
> On Thu, 20 May 2010, Jan-Bernd Themann wrote:
>>>> Thought more about that. The case at hand (ehea) is nasty:
>>>>
>>>> The driver does _NOT_ disable the rx interrupt in the card in the rx
>>>> interrupt handler - for whatever reason.
>>>
>>> Yeah I saw that, but I don't know why it's written that way. Perhaps
>>> Jan-Bernd or Doug will chime in and enlighten us? :)
>>
>>  From our perspective there is no need to disable interrupts for the
>> RX side as the chip does not fire further interrupts until we tell
>> the chip to do so for a particular queue. We have multiple receive
>
> The traces tell a different story though:
>
>      ehea_recv_irq_handler()
>        napi_reschedule()
>      eoi()
>      ehea_poll()
>        ...
>        ehea_recv_irq_handler()<---------------- ???
>          napi_reschedule()
>        ...
>        napi_complete()
>
> Can't tell whether you can see the same behaviour in mainline, but I
> don't see a reason why not.

I was going to suggest that because these are threaded handlers, perhaps 
they are rescheduled on a different CPU and then receive the interrupt 
for the other CPU/queue that Jan was mentioning.

But, the handlers are affined if I remember correctly, and we aren't 
running with multiple receive queues. So, we're back to the same 
question, why are we seeing another irq. It comes in before 
napi_complete() and therefor before the ehea_reset*() block of calls 
which do the equivalent of re-enabling interrupts.

--
Darren

>
>> queues with an own interrupt each so that the interrupts can arrive
>> on multiple CPUs in parallel.  Interrupts are enabled again when we
>> leave the NAPI Poll function for the corresponding receive queue.
>
> I can't see a piece of code which does that, but that's probably just
> lack of detailed hardware knowledge on my side.
>
> Thanks,
>
> 	tglx


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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