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

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Date:	Thu, 20 May 2010 16:45:54 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhltc@...ibm.com>
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 Thu, 20 May 2010, Darren Hart wrote:

> 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.

Can you slap a few trace points into that driver with a stock mainline
kernel and verify that ?

Thanks,

	tglx
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