Re: [PATCH 2/3] x86: x2apic/cluster: Make use of lowest priority delivery mode

[Yinghai Lu <yinghai@kernel.org>]

On Mon, May 21, 2012 at 4:02 PM, Yinghai Lu <yinghai@kernel.org> wrote:
> On Mon, May 21, 2012 at 1:13 AM, Alexander Gordeev <agordeev@redhat.com> wrote:
>> On Sat, May 19, 2012 at 01:53:36PM -0700, Yinghai Lu wrote:
>>> > +static void
>>> > +x2apic_cluster_vector_allocation_domain(int cpu, struct cpumask *retmask)
>>> > +{
>>> > +       cpumask_copy(retmask, cpu_possible_mask);
>>>
>>> why not using per_cpu(cpus_in_cluster, cpu) instead?
>>
>> Because it would lead to suboptimal results when updating IRQ affinity:
>>
>> int __ioapic_set_affinity(struct irq_data *data, const struct cpumask *mask,
>>                          unsigned int *dest_id)
>> {
>>        struct irq_cfg *cfg = data->chip_data;
>>
>>        if (!cpumask_intersects(mask, cpu_online_mask))
>>                return -1;
>>
>>        if (assign_irq_vector(data->irq, data->chip_data, mask))
>>                return -1;
>>
>> This call ^^^ will update cfg->domain with the value returned by the call to
>> apic->vector_allocation_domain(). If per_cpu(cpus_in_cluster, cpu) is returned
>> as cfg->domain here then all other clusters contained in the 'mask' will not
>> be taken into consideration by the apic->cpu_mask_to_apicid_and() call below.
>>
>>        cpumask_copy(data->affinity, mask);
>>
>>        *dest_id = apic->cpu_mask_to_apicid_and(mask, cfg->domain);
>>
>> So we really need to submit all possible CPUs here ^^^ to be able finding the
>> best/heaviest cluster out of the 'mask'.
>
> ok.

Maybe you can not simply to change that to possible_cpu_mask.

otherwise assign_irq_vector will try to get same vector for all cpus
for one irq.

then will make x2apic cluster mode will one support 224 irqs instead.

so you need to make vector_allocation_domain() to be less cpu set.

Yinghai