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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.05.2024 17:15, Roger Pau Monné wrote:
> 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."

Fine with me. Looks like simply deleting "creating" and keeping the rest
as it was would be okay too? Whatever you prefer in the end.

>>> --- 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.

That was the conclusion I came to, or else I wouldn't have made the
suggestion. Unless you have a strong preference for the as-is form, I'd
indeed prefer the suggested alternative.

Jan