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
