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

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

[Paul E. McKenney]

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

------------------------------------------------------------------------

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bc4f7a9ba64e6..36a9399be331d 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1514,6 +1514,7 @@ int remove_cpu(unsigned int cpu)
 
 	lock_device_hotplug();
 	ret = device_offline(get_cpu_device(cpu));
+	flush_workqueue(system_unbound_wq);
 	unlock_device_hotplug();
 
 	return ret;
@@ -1730,6 +1731,7 @@ int add_cpu(unsigned int cpu)
 
 	lock_device_hotplug();
 	ret = device_online(get_cpu_device(cpu));
+	flush_workqueue(system_unbound_wq);
 	unlock_device_hotplug();
 
 	return ret;

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

[Waiman Long]

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?

Thanks,
Longman

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

[Paul E. McKenney]

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>

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

[Waiman Long]

On 3/18/26 2:43 PM, 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 <paulmck@kernel.org>
>
Thanks for the confirmation.

Cheers,
Longman

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

[Paul E. McKenney]

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 <paulmck@kernel.org>

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

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

[Waiman Long]

On 3/24/26 5:41 AM, Paul E. McKenney wrote:
> 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 <paulmck@kernel.org>
> 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?

Thank for the notice. I haven't much testing with respect to PREEMPT_RT 
kernel. I will try to run some tests on PREEMPT_RT kernel and see if 
there is any problem. Please let me know if you found out the new cpuset 
code is at fault after bisection.

Cheers,
Longman