linux-kernel - Re: CPU hotplug and chained interrupts on x86

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Date:	Fri, 2 Oct 2015 01:07:28 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CPU hotplug and chained interrupts on x86

On 2015/10/1 22:31, Thomas Gleixner wrote:
> On Thu, 1 Oct 2015, Mika Westerberg wrote:
>> Now if I plug/unplug the card I may get few interrupts to CPU0 but rest
>> of the interrupts never happen. Probably because IO-APIC forwards them
>> to the lowest priority CPU which is offline at this point.
>>
>> There is following check in fixup_irqs():
>>
>> 	if (!irq_has_action(irq) || irqd_is_per_cpu(data) ||
>> 	    cpumask_subset(affinity, cpu_online_mask)) {
>> 		raw_spin_unlock(&desc->lock);
>> 		continue;
>> 	}   
>>
>> If an interrupt is requested by a driver it will force new affinity and
>> everything works fine. However if the interrupt is chained (it does not
>> have ->action) this is skipped and the current affinity remains.
>>
>> We could detect here if the interrupt is chained but there seems to be
>> no easy way to determine it currently so we would need to add a new flag
>> to desc->status_use_accessors that gets set in __irq_do_set_handler()
>> when is_chained is 1.
> 
> Either there or in irq_data. Need to look at it in detail.
Currently  we have no flag for chained, I suggested to add one dedicated
flag for it.

>  
>> Alternative I could implement ->irq_set_affinity() in the GPIO driver in
>> question [1] which always calls directly parent chip's ->irq_set_affinity()
>> but I'm not sure if that is allowed.
> 
> I rather prefer to avoid that.
We should report chained state and parent irq, so user and irqbalance
may make smarter decision based on those info.
Thanks!
Gerry

> 
> Thanks,
> 
> 	tglx
> 
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