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. Jan