Re: [linux-next:master] [mm/hugetlb_vmemmap] 875fa64577: vm-scalability.throughput -34.3% regression

[Christian Borntraeger]

Am 17.07.24 um 09:52 schrieb Janosch Frank:
> On 7/9/24 07:11, kernel test robot wrote:
> > Hello,
> >
> > kernel test robot noticed a -34.3% regression of vm-scalability.throughput on:
> >
> >
> > commit: 875fa64577da9bc8e9963ee14fef8433f20653e7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >
> > [still regression on linux-next/master 0b58e108042b0ed28a71cd7edf5175999955b233]
> This has hit s390 huge page backed KVM guests as well.
> Our simple start/stop test case went from ~5 to over 50 seconds of runtime.

Could this be one of the synchronize_rcu calls? This patch adds lots of them. On s390 with HZ=100 those are really expensive.