On Wed, Mar 18, 2026 at 11:43:37AM -0700, Paul E. McKenney wrote:
> On Wed, Mar 18, 2026 at 11:02:16AM -0400, Waiman Long wrote:
> > On 3/18/26 8:53 AM, Paul E. McKenney wrote:
> > > Hello!
> > >
> > > Running rcutorture on v7.0-rc3 results in spurious CPU-hotplug failures,
> > > most frequently on the TREE03 scenario, which suffers about ten such
> > > failures per hundred hours of test time. Repeat-by is as follows:
> > >
> > > tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 80 --duration 100h
> > > --configs "100*TREE03" --trust-make
> > >
> > > Though a faster repeat-by instead uses kvm-remote.sh and lots of systems.
> > >
> > > Bisection converges here:
> > >
> > > 6df415aa46ec ("cgroup/cpuset: Defer housekeeping_update() calls from CPU
> > > hotplug to workqueue")
> > >
> > > Reverting this commit gets rid of the spurious CPU-hotplug failures.
> > > Of course, this also gets rid of some ability to do dynamic nohz_full
> > > processing.
> > >
> > > Now, the problem might be that the workqueue handler might still be
> > > in flight by the time that rcutorture fired up the next CPU-hotplug
> > > operation, especially given that the TREE03 scenario only waits 200
> > > milliseconds between these operations. This suggests waiting for this
> > > handler before ending each CPU-hotplug operation. And the crude patch
> > > below does make the problem go away.
> > >
> > > This alleged fix is quite heavy-handed, and also fragile in that if
> > > hk_sd_workfn() uses a different workqueue, this breaks. It might be
> > > better to call into the cgroups/cpusets code and to use flush_work()
> > > to wait only on hk_sd_workfn() and nothing else. But it seemed best to
> > > keep things trivial to start with.
> > >
> > > Either way, please consider the patch below to be part of this bug report
> > > rather than a proper fix.
> > >
> > > Thoughts?
> > >
> > > Thanx, Paul
> > There is a fix commit ca174c705db5 ("cgroup/cpuset: Call
> > rebuild_sched_domains() directly in hotplug") in rc4 that may help. Could
> > you try out the rc4 kernel to see if that can resolve the problem that you
> > have?
>
> It does, thank you!
>
> Tested-by: Paul E. McKenney <[email protected]>
This did fix the problem, except for PREEMPT_RT kernels (which I have
not yet bisected). If there is another patch for that configuration,
could you please let me know?
Thanx, Paul
