Re: [Xen-devel] [PATCH v3 07/15] x86/IRQ: target online CPUs when binding guest IRQ

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, May 17, 2019 at 04:48:21AM -0600, Jan Beulich wrote:
> fixup_irqs() skips interrupts without action. Hence such interrupts can
> retain affinity to just offline CPUs. With "noirqbalance" in effect,
> pirq_guest_bind() so far would have left them alone, resulting in a non-
> working interrupt.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v3: New.
> ---
> I've not observed this problem in practice - the change is just the
> result of code inspection after having noticed action-less IRQs in 'i'
> debug key output pointing at all parked/offline CPUs.
> 
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -1683,9 +1683,27 @@ int pirq_guest_bind(struct vcpu *v, stru
>  
>          desc->status |= IRQ_GUEST;
>  
> -        /* Attempt to bind the interrupt target to the correct CPU. */
> -        if ( !opt_noirqbalance && (desc->handler->set_affinity != NULL) )
> -            desc->handler->set_affinity(desc, cpumask_of(v->processor));
> +        /*
> +         * Attempt to bind the interrupt target to the correct (or at least
> +         * some online) CPU.
> +         */
> +        if ( desc->handler->set_affinity )
> +        {
> +            const cpumask_t *affinity = NULL;
> +
> +            if ( !opt_noirqbalance )
> +                affinity = cpumask_of(v->processor);
> +            else if ( !cpumask_intersects(desc->affinity, &cpu_online_map) )
> +            {
> +                cpumask_setall(desc->affinity);
> +                affinity = &cpumask_all;
> +            }
> +            else if ( !cpumask_intersects(desc->arch.cpu_mask,
> +                                          &cpu_online_map) )

I'm not sure I see the purpose of the desc->arch.cpu_mask check,
wouldn't it be better to just use else and set the affinity to
desc->affinity?

Or it's just an optimization to avoid doing the set_affinity call if
the interrupt it already bound to an online CPU?

Thanks, Roger.

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