Re: [PATCH for-4.19 1/9] x86/irq: remove offline CPUs from old CPU mask when adjusting move_cleanup_count

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

On Wed, May 29, 2024 at 02:40:51PM +0200, Jan Beulich wrote:
> On 29.05.2024 11:01, Roger Pau Monne wrote:
> > When adjusting move_cleanup_count to account for CPUs that are offline also
> > adjust old_cpu_mask, otherwise further calls to fixup_irqs() could subtract
> > those again creating and create an imbalance in move_cleanup_count.
> 
> I'm in trouble with "creating"; I can't seem to be able to guess what you may
> have meant.

Oh, sorry, that's a typo.

I was meaning to point out that not removing the already subtracted
CPUs from the mask can lead to further calls to fixup_irqs()
subtracting them again and move_cleanup_count possibly underflowing.

Would you prefer to write it as:

"... could subtract those again and possibly underflow move_cleanup_count."

> > Fixes: 472e0b74c5c4 ('x86/IRQ: deal with move cleanup count state in 
> > fixup_irqs()')
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> With the above clarified (adjustment can be done while committing)
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> > --- a/xen/arch/x86/irq.c
> > +++ b/xen/arch/x86/irq.c
> > @@ -2572,6 +2572,14 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
> >              desc->arch.move_cleanup_count -= cpumask_weight(affinity);
> >              if ( !desc->arch.move_cleanup_count )
> >                  release_old_vec(desc);
> > +            else
> > +                /*
> > +                 * Adjust old_cpu_mask to account for the offline CPUs,
> > +                 * otherwise further calls to fixup_irqs() could subtract 
> > those
> > +                 * again and possibly underflow the counter.
> > +                 */
> > +                cpumask_and(desc->arch.old_cpu_mask, 
> > desc->arch.old_cpu_mask,
> > +                            &cpu_online_map);
> >          }
> 
> While functionality-wise okay, imo it would be slightly better to use
> "affinity" here as well, so that even without looking at context beyond
> what's shown here there is a direct connection to the cpumask_weight()
> call. I.e.
> 
>                 cpumask_andnot(desc->arch.old_cpu_mask, 
> desc->arch.old_cpu_mask,
>                                affinity);
> 
> Thoughts?

It was more straightforward for me to reason that removing the offline
CPUs is OK, but I can see that you might prefer to use 'affinity',
because that's the weight that's subtracted from move_cleanup_count.
Using either should lead to the same result if my understanding is
correct.

Thanks, Roger.