Re: [BUG] cgroups/cpusets: Spurious CPU-hotplug failures

["Paul E. McKenney" <paulmck@kernel.org>]

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 <paulmck@kernel.org>