Subject: [PATCH] arm: use cpu_online_mask when using forced irq_set_affinity

From: [email-masked] (Sudeep Holla)
Date: Thu, 17 Jul 2014 10:58:27 +0100

Hi Thomas,

On 20/06/14 16:16, Russell King - ARM Linux wrote:
> On Fri, Jun 20, 2014 at 02:04:52PM +0100, Sudeep Holla wrote:
>> Hi Russell,
>>
>> On 23/05/14 13:51, Sudeep Holla wrote:
>>>
>>>
>>> On 23/05/14 13:10, Russell King - ARM Linux wrote:
>>>> On Fri, May 09, 2014 at 05:40:40PM +0100, Sudeep Holla wrote:
>>>>> From: Sudeep Holla <[email-masked]>
>>>>>
>>>>> Commit 01f8fa4f01d8("genirq: Allow forcing cpu affinity of interrupts")
>>>>> enabled the forced irq_set_affinity which previously refused to route an
>>>>> interrupt to an offline cpu.
>>>>>
>>>>> Commit ffde1de64012("irqchip: Gic: Support forced affinity setting")
>>>>> implements this force logic and disables the cpu online check for GIC
>>>>> interrupt controller.
>>>>>
>>>>> When __cpu_disable calls migrate_irqs, it disables the current cpu in
>>>>> cpu_online_mask and uses forced irq_set_affinity to migrate the IRQs
>>>>> away from the cpu but passes affinity mask with the cpu being offlined
>>>>> also included in it.
>>>>>
>>>>> When calling irq_set_affinity with force == true in a cpu hotplug path,
>>>>> the caller must ensure that the cpu being offlined is not present in the
>>>>> affinity mask or it may be selected as the target CPU, leading to the
>>>>> interrupt not being migrated.
>>>>>
>>>>> This patch uses cpu_online_mask when using forced irq_set_affinity so
>>>>> that the IRQs are properly migrated away.
>>>>>
>>>>> Tested on TC2 hotpluging CPU0 in and out. Without this patch the system
>>>>> locks up as the IRQs are not migrated away from CPU0.
>>>>
>>>> You don't explain /how/ this happens, and I'm not convinced that you've
>>>> properly diagnosed this bug.
>>>>
>>>
>>> Sorry for not being elaborate enough.
>>> - On boot by default all the irqs have cpu_online_mask as affinity
>>> - Now if CPU0 is being hotplugged out, CPU0 is removed from cpu_online_mask
>>>      and migrate_irqs is called
>>> - In migrate_one_irq, when affinity is read from the irq_desc, it still contains
>>>      CPU0 which is expected.
>>> - irq_set_affinity is called with affinity with CPU0 set and force = true,
>>>      which chooses CPU0 resulting in not migrating the IRQ.
>>>
>>>>> @@ -155,11 +155,15 @@ static bool migrate_one_irq(struct irq_desc *desc)
>>>>>     	if (irqd_is_per_cpu(d) || !cpumask_test_cpu(smp_processor_id(), affinity))
>>>>>     		return false;
>>>>>
>>>>> -	if (cpumask_any_and(affinity, cpu_online_mask) >= nr_cpu_ids) {
>>>>> -		affinity = cpu_online_mask;
>>>>> +	if (cpumask_any_and(affinity, cpu_online_mask) >= nr_cpu_ids)
>>>>>     		ret = true;
>>>>> -	}
>>>>
>>>> The idea here with the original code is:
>>>>
>>>> - if the current CPU (which is the one being offlined) is not in the
>>>>      affinity mask, do nothing.
>>>> - if "affinity & cpu_online_mask" indicates that there's no CPUs in the
>>>>      new set (cpu_online_mask must have been updated to indicate that the
>>>>      current CPU is offline) then re-set the affinity mask and report that
>>>>      we forced a change.
>>>> - otherwise, re-set the existing affinity (which will force the IRQ
>>>>      controller to re-evaluate it's routing given the affinity and online
>>>>      CPUs.)
>>>>
>>>
>>> I completely understand the above idea, except that the new feature added
>>> to allow forced affinity setting(as mentioned in the commit log by 2 commits),
>>> changes the behaviour of last step.
>>>
>>> IRQ controller now re-evaluates it's routing based on the given affinity alone
>>> and doesn't consider online CPUs when force = true is set. This will result in
>>> the CPU being offlined chosen as the target if it happens to be the first in the
>>> affinity mask.
>>>
>>>> This code is correct.  In fact, changing it as you have, you /always/
>>>> reset the affinity mask whether or not the CPU being offlined is the
>>>> last CPU in the affinity set.
>>>>
>>>> If you are finding that CPU0 is left with interrupts afterwards, the
>>>> bug lies elsewhere - probably in the IRQ controller code.
>>>>
>>>
>>> Since the IRQ controller code is changed to provide that feature, either
>>> - we have to choose not to use forced option, or
>>> - we need to make sure we pass valid affinity mask with force = true option
>>>
>>> I chose latter in this patch. Let me know your opinion.
>>>
>>
>> Any suggestions on this ? Since commit 01f8fa4f01d8 and ffde1de64012 are now in
>> stable releases, CPU0 hotplug is broken there now.
>
> Maybe we should ask Thomas, as he's (a) the maintainer of the irqchip
> stuff, and (b) the author of the patch causing the breakage.
>
>  From what I can see looking at the x86 code, the work-around in
> ffde1de64012 is wrong.
>

Can provide your thoughts on how to solve this issue ?

Is it expected from all the irqchip implementation to use force flag in
irq_set_affinity to ignore cpu_online_mask similar to GIC ?

Regards,
Sudeep