Re: [PATCH v2 6/7] x86/irq: handle moving interrupts in _assign_irq_vector()

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

On Tue, Jun 11, 2024 at 03:18:32PM +0200, Jan Beulich wrote:
> On 10.06.2024 16:20, 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,
> 
> I'm having trouble relating this (->arch.old_cpu_mask related) to ...
> 
> > move the interrupt to a different CPU part of
> > the provided mask
> 
> ... this (->arch.cpu_mask related).

No, the "provided mask" here is the "mask" parameter, not
->arch.cpu_mask.

> 
> > and keep the current ->arch.old_{cpu_mask,vector} for the
> > pending interrupt movement to be completed.
> 
> Right, that's to clean up state from before the initial move. What isn't
> clear to me is what's to happen with the state of the intermediate
> placement. Description and code changes leave me with the impression that
> it's okay to simply abandon, without any cleanup, yet I can't quite figure
> why that would be an okay thing to do.

There isn't much we can do with the intermediate placement, as the CPU
is going offline.  However we can drain any pending interrupts from
IRR after the new destination has been set, since setting the
destination is done from the CPU that's the current target of the
interrupts.  So we can ensure the draining is done strictly after the
target has been switched, hence ensuring no further interrupts from
this source will be delivered to the current CPU.

> > --- a/xen/arch/x86/irq.c
> > +++ b/xen/arch/x86/irq.c
> > @@ -544,7 +544,53 @@ 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;
> > +            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. */
> > +            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;
> > +
> > +            return 0;
> > +        }
> 
> In how far is this guaranteed to respect the (new) affinity that was set,
> presumably having led to the movement in the first place?

The 'mask' parameter should account for the new affinity, hence the
cpumask_intersects() check guarantees we are moving to a CPU still in
the affinity mask.

> > @@ -600,7 +646,17 @@ next:
> >          current_vector = vector;
> >          current_offset = offset;
> >  
> > -        if ( valid_irq_vector(old_vector) )
> > +        if ( desc->arch.move_in_progress || desc->arch.move_cleanup_count )
> > +        {
> > +            ASSERT(!cpumask_intersects(desc->arch.cpu_mask, 
> > &cpu_online_map));
> > +            /*
> > +             * Special case when evacuating an interrupt from a CPU to be
> > +             * offlined and the interrupt was already in the process of 
> > being
> > +             * moved.  Leave ->arch.old_{vector,cpu_mask} as-is and just
> > +             * replace ->arch.{cpu_mask,vector} with the new destination.
> > +             */
> 
> And where's the cleaning up of ->arch.old_* going to be taken care of then?

Such cleaning will be handled normally by the interrupt still having
->arch.move_{in_progress,cleanup_count} set.  The CPUs in
->arch.old_cpu_mask must not all be offline, otherwise the logic in
fixup_irqs() would have already released the old vector.

Thanks, Roger.