linux-kernel - Re: [RFC PATCH v1 2/4] irqchip: GICv3: set non-percpu irqs status with _IRQ_MOVE

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Message-ID: <55EDA5A5.9070805@arm.com>
Date:	Mon, 07 Sep 2015 15:56:37 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Yang Yingliang <yangyingliang@...wei.com>
CC:	Jiang Liu <jiang.liu@...ux.intel.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Mark Rutland <mark.rutland@....com>,
	Will Deacon <will.deacon@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Hanjun Guo <hanjun.guo@...aro.org>
Subject: Re: [RFC PATCH v1 2/4] irqchip: GICv3: set non-percpu irqs status
 with _IRQ_MOVE_PCNTXT

Hi Thomas,

On 07/09/15 14:24, Thomas Gleixner wrote:
> On Mon, 7 Sep 2015, Marc Zyngier wrote:
>> On 06/09/15 06:56, Jiang Liu wrote:
>>> On 2015/9/6 12:23, Yang Yingliang wrote:
>>>> Use irq_settings_set_move_pcntxt() helper irqs status with
>>>> _IRQ_MOVE_PCNTXT. So that it can do set affinity when calling
>>>> irq_set_affinity_locked().
>>> Hi Yingliang,
>>> 	We could only set _IRQ_MOVE_PCNTCT flag to enable migrating
>>> IRQ in process context if your hardware platform supports atomically
>>> change IRQ configuration. Not sure whether that's true for GICv3.
>>> If GICv3 doesn't support atomically change irq configuration, this
>>> change may cause trouble.
>>
>> I think it boils down to what exactly "process context" means here. If
>> this means "we do not need to mask the interrupt" while moving it, then
>> it should be fine (the GIC architecture guarantees that a pending
>> interrupt will be migrated).
>>
>> Is there any other requirement for this flag?
> 
> The history of this flag is as follows:
> 
> On x86 interrupts can only be safely migrated while the interrupt is
> handled.

Woa! That's creative! :-) I suppose this doesn't work very well with CPU
hotplug though...

> With the introduction of IRQ remapping this requirement
> changed. Remapped interrupts can be migrated in any context.
> 
> If you look at irq_set_affinity_locked()
> 
>    if (irq_can_move_pcntxt(data) {
>       irq_do_set_affinity(data,...)
>         chip->irq_set_affinity(data,...);
>    } else {
>       irqd_set_move_pending(data);
>    }
> 
> So if IRQ_MOVE_PCNTXT is not set, we handle the migration of the
> interrupt from next the interrupt. If it's set set_affinity() is
> called right away.

OK, that is now starting to make more sense.

> All architectures which do not select GENERIC_PENDING_IRQ are using
> the direct method.

Right. On ARM, only the direct method makes sense so far (we have no
constraint such as the one you describe above).

So I wonder why we bother introducing the IRQ_MOVE_PCNTXT flag on ARM at
all. Is that just because migration.c is only compiled when
GENERIC_PENDING_IRQ is set?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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