linux-kernel - Re: IRQ affinity enforced only after first interrupt.

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Date:	Wed, 4 Apr 2012 12:01:45 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Jiang Liu <liuj97@...il.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yael Shenhav <yaeli@...lanox.com>
Subject: Re: IRQ affinity enforced only after first interrupt.

On Mon, Mar 26, 2012 at 09:04:20PM +0200, Thomas Gleixner wrote:
> On Mon, 26 Mar 2012, Yevgeny Petrilin wrote:
> 
> > > 
> > > The architecture specific code will determine whether the IRQ could be migrated
> > > in process context. For example, the IRQ_MOVE_PCNTXT flag will be set on x86
> > > systems if interrupt remapping is enabled.
> >
> > Actually I am encountering this issue with x86, and see different
> > behavior with different HW devices (NICs). On same machine I have
> > one device that responds immediately to affinity changes while the
> > other one changes the affinity only after first interrupt.
> 
> That simply depends on the underlying hardware. On certain hardware we
> can change the affinity only in hard interrupt context, that means
> right when a interrupt of that device is delivered.
> 
> On the other devices we can change it right away and the corresponding
> interrupt chips set IRQ_MOVE_PCNTXT to indicate that.

Actually, even with IRQ_MOVE_PCNTXT capable chips, a hardware handler still
might be called on a core that belongs to old affinity, after the successful
write of new affinity. Threaded handlers are also racy with irq affinity
updates.

If that is inconsistency, bug or design?

> There is nothing we can do about this. It's dictated by hardware.

May be we could wait for desc->pending_mask to be cleared before returning from
irq_set_affinity()?

> Thanks,
> 
> 	tglx
> --
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-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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