Re: [PATCH 1/2] x86/irq: fix calculation of max PV dom0 pIRQs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.11.2024 09:01, Roger Pau Monné wrote:
> On Thu, Nov 21, 2024 at 12:39:06PM +0100, Jan Beulich wrote:
>> On 21.11.2024 12:04, Roger Pau Monné wrote:
>>> On Thu, Nov 21, 2024 at 11:49:44AM +0100, Jan Beulich wrote:
>>>> On 20.11.2024 12:35, Roger Pau Monne wrote:
>>>>> The current calculation of PV dom0 pIRQs uses:
>>>>>
>>>>> n = min(fls(num_present_cpus()), dom0_max_vcpus());
>>>>>
>>>>> The usage of fls() is wrong, as num_present_cpus() already returns the 
>>>>> number
>>>>> of present CPUs, not the bitmap mask of CPUs.
>>>>
>>>> Hmm. Perhaps that use of fls() should have been accompanied by a comment, 
>>>> but
>>>> I think it might have been put there intentionally, to avoid linear growth.
>>>> Which isn't to say that I mind the adjustment, especially now that we don't
>>>> use any clustered modes anymore for I/O interrupts. I'm merely questioning
>>>> the Fixes: tag, and with that whether / how far to backport.
>>>
>>> Hm, sorry I've assumed the fls() was a typo.  It seems wrong to cap
>>> dom0 vCPUs with the fls of the present CPUs number.  For consistency,
>>> if the intention was to use fls to limit growth, I would have expected
>>> to also be applied to the dom0 number of vCPUs.
>>
>> FTR: My vague recollection (it has been nearly 10 years) is that I first had
>> it there, too. Until I realized that it hardly ever would have any effect,
>> because of the min(). And for Dom0-s with extremely few vCPU-s it seemed
>> reasonable to not apply that cap there.
> 
> I don't have a strong opinion regarding the fixes tag, but would like
> to get this sorted as soon as possible.  If you prefer just drop the
> fixes tag.  I think this wants backporting to all supported releases,
> because it's an issue XenServer has hit on real servers when dom0 is
> sized to use 16 vCPUs at most.

In which case yes, I guess we want to keep the Fixes:.

Jan