Re: [PATCH v3 for-4.19 2/3] x86/irq: handle moving interrupts in _assign_irq_vector()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.06.2024 18:56, Roger Pau Monne wrote:
> Currently there's logic in fixup_irqs() that attempts to prevent
> _assign_irq_vector() from failing, as fixup_irqs() is required to evacuate all
> interrupts from the CPUs not present in the input mask.  The current logic in
> fixup_irqs() is incomplete, as it doesn't deal with interrupts that have
> move_cleanup_count > 0 and a non-empty ->arch.old_cpu_mask field.
> 
> Instead of attempting to fixup the interrupt descriptor in fixup_irqs() so 
> that
> _assign_irq_vector() cannot fail, introduce logic in _assign_irq_vector()
> to deal with interrupts that have either move_{in_progress,cleanup_count} set
> and no remaining online CPUs in ->arch.cpu_mask.
> 
> If _assign_irq_vector() is requested to move an interrupt in the state
> described above, first attempt to see if ->arch.old_cpu_mask contains any 
> valid
> CPUs that could be used as fallback, and if that's the case do move the
> interrupt back to the previous destination.  Note this is easier because the
> vector hasn't been released yet, so there's no need to allocate and setup a 
> new
> vector on the destination.
> 
> Due to the logic in fixup_irqs() that clears offline CPUs from
> ->arch.old_cpu_mask (and releases the old vector if the mask becomes empty) it
> shouldn't be possible to get into _assign_irq_vector() with
> ->arch.move_{in_progress,cleanup_count} set but no online CPUs in
> ->arch.old_cpu_mask.
> 
> However if ->arch.move_{in_progress,cleanup_count} is set and the interrupt 
> has
> also changed affinity, it's possible the members of ->arch.old_cpu_mask are no
> longer part of the affinity set, move the interrupt to a different CPU part of
> the provided mask and keep the current ->arch.old_{cpu_mask,vector} for the
> pending interrupt movement to be completed.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -544,7 +544,58 @@ static int _assign_irq_vector(struct irq_desc *desc, 
> const cpumask_t *mask)
>      }
>  
>      if ( desc->arch.move_in_progress || desc->arch.move_cleanup_count )
> -        return -EAGAIN;
> +    {
> +        /*
> +         * If the current destination is online refuse to shuffle.  Retry 
> after
> +         * the in-progress movement has finished.
> +         */
> +        if ( cpumask_intersects(desc->arch.cpu_mask, &cpu_online_map) )
> +            return -EAGAIN;
> +
> +        /*
> +         * Due to the logic in fixup_irqs() that clears offlined CPUs from
> +         * ->arch.old_cpu_mask it shouldn't be possible to get here with
> +         * ->arch.move_{in_progress,cleanup_count} set and no online CPUs in
> +         * ->arch.old_cpu_mask.
> +         */
> +        ASSERT(valid_irq_vector(desc->arch.old_vector));
> +        ASSERT(cpumask_intersects(desc->arch.old_cpu_mask, &cpu_online_map));
> +
> +        if ( cpumask_intersects(desc->arch.old_cpu_mask, mask) )
> +        {
> +            /*
> +             * Fallback to the old destination if moving is in progress and 
> the
> +             * current destination is to be offlined.  This is only possible 
> if
> +             * the CPUs in old_cpu_mask intersect with the affinity mask 
> passed
> +             * in the 'mask' parameter.
> +             */
> +            desc->arch.vector = desc->arch.old_vector;

I'm a little puzzled that you use desc->arch.old_vector here, but ...

> +            cpumask_and(desc->arch.cpu_mask, desc->arch.old_cpu_mask, mask);
> +
> +            /* Undo any possibly done cleanup. */
> +            for_each_cpu(cpu, desc->arch.cpu_mask)
> +                per_cpu(vector_irq, cpu)[desc->arch.vector] = irq;
> +
> +            /* Cancel the pending move and release the current vector. */
> +            desc->arch.old_vector = IRQ_VECTOR_UNASSIGNED;
> +            cpumask_clear(desc->arch.old_cpu_mask);
> +            desc->arch.move_in_progress = 0;
> +            desc->arch.move_cleanup_count = 0;
> +            if ( desc->arch.used_vectors )
> +            {
> +                ASSERT(test_bit(old_vector, desc->arch.used_vectors));
> +                clear_bit(old_vector, desc->arch.used_vectors);

... old_vector here. Since we have the latter, uniformly using it might
be more consistent. I realize though that irq_to_vector() has cases where
it wouldn't return desc->arch.old_vector; I think, however, that in those
case we can't make it here. Still I'm not going to insist on making the
adjustment. Happy to make it though while committing, should you agree.

Also I'm not happy to see another instance of this pattern appear. In
x86-specific code this is inefficient, as {set,clear}_bit resolve to the
same insn as test_and_{set,clear}_bit(). Therefore imo more efficient
would be

                if (!test_and_clear_bit(old_vector, desc->arch.used_vectors))
                    ASSERT_UNREACHABLE();

(and then the two if()s folded).

I've been meaning to propose a patch to the other similar sites ...

Jan