linux-kernel - Re: [PATCH] x86: Add check for number of available vectors before CPU down

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Message-ID: <52B34313.8050905@redhat.com>
Date:	Thu, 19 Dec 2013 14:03:47 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	rui wang <ruiv.wang@...il.com>
CC:	linux-kernel@...r.kernel.org, x86@...nel.org,
	"Chen, Gong" <gong.chen@...el.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>
Subject: Re: [PATCH] x86: Add check for number of available vectors before
 CPU down



On 12/19/2013 02:19 AM, rui wang wrote:
> On 12/19/13, Prarit Bhargava <prarit@...hat.com> wrote:
>>
>>
>> On 12/03/2013 09:48 PM, rui wang wrote:
>>> On 11/20/13, Prarit Bhargava <prarit@...hat.com> wrote:
>>> Have you considered the case when an IRQ is destined to more than one CPU?
>>> e.g.:
>>>
>>> bash# cat /proc/irq/89/smp_affinity_list
>>> 30,62
>>> bash#
>>>
>>> In this case offlining CPU30 does not seem to require an empty vector
>>> slot. It seems that we only need to change the affinity mask of irq89.
>>> Your check_vectors() assumed that each irq on the offlining cpu
>>> requires a new vector slot.
>>>
>>
>> Rui,
>>
>> The smp_affinity_list only indicates a preferred destination of the IRQ, not
>> the
>> *actual* location of the CPU.  So the IRQ is on one of cpu 30 or 62 but not
>> both
>> simultaneously.
>>
> 
> It depends on how IOAPIC (or MSI/MSIx) is configured. An IRQ can be
> simultaneously broadcast to all destination CPUs (Fixed Mode) or
> delivered to the CPU with the lowest priority task (Lowest Priority
> Mode). It's programmed in the Delivery Mode bits of the IOAPIC's IO
> Redirection table registers, or the Message Data Register in the case
> of MSI/MSIx
> 

Thanks for clueing me in Rui :).  You're right.  I do need to do a

                        if (irq_has_action(irq) || !irqd_is_per_cpu(data) ||
                            !cpumask_subset(affinity, cpu_online_mask))

instead of just

			if  (!irqd_is_per_cpu(data))

I'm going to do some additional testing tonight across various systems and will
repost tomorrow if the testing is successful.

P.
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