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

[Jiang Liu]

On 2015/9/7 22:56, Marc Zyngier wrote:
> 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...
X86 has special handling of this case when hot-removing a CPU.
Basically, it does:
1) mask an irq
2) migrate irq to other cpus with set_affinity
3) redirect(retrigger) irq to other CPUs if it's pending on the CPU to
be removed.
Thanks!
Gerry

> 
>> 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.
> 
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