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SubjectRe: [RFC] Correct behaviour of irq affinity?
On Mon, Mar 23, 2009 at 10:49 PM, Rusty Russell <> wrote:
> The effect of setting desc->affinity (ie. from userspace via sysfs) has varied
> over time.  In 2.6.27, the 32-bit code anded the value with cpu_online_map,
> and both 32 and 64-bit did that anding whenever a cpu was unplugged.
> 2.6.29 consolidated this into one routine (and fixed hotplug) but introduced
> another variation: anding the affinity with cfg->domain.  Is this right, or
> should we just set it to what the user said?  Or as now, indicate that we're
> restricting it.
> If we should change it, here's what the patch looks like against x86 tip
> (cpu_mask_to_apicid_and already takes cpu_online_mask into account):
> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
> index 86827d8..30906cd 100644
> --- a/arch/x86/kernel/apic/io_apic.c
> +++ b/arch/x86/kernel/apic/io_apic.c
> @@ -592,10 +592,10 @@ set_desc_affinity(struct irq_desc *desc, const struct cpumask *mask)
>        if (assign_irq_vector(irq, cfg, mask))
>                return BAD_APICID;
> -       cpumask_and(desc->affinity, cfg->domain, mask);
> +       cpumask_copy(desc->affinity, mask);
>        set_extra_move_desc(desc, mask);
> -       return apic->cpu_mask_to_apicid_and(desc->affinity, cpu_online_mask);
> +       return apic->cpu_mask_to_apicid_and(desc->affinity, cfg->domain);
>  }
>  static void
cfg->domain for logical flat: will be ALL_CPUS
for phys flat (aka bigsmp on 32bit) will be one cpu set mask.

so desc->affinity: for logical will be not changed, but
set_desc_affinity() return will be changed. ( not add with
cpu_online_mask anymore)
when mask is 0x0f
for phys flat, desc->affinity will be changed to 0x0f from
0x01/0x02/0x04/08, return set_desc_affinity is not changed.
so /proc/irq/xx/smp_affinity will be changed. and it does reflect that
actually affinity.

so this patch looks not right.

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