Re: [PATCHv4 2/2] sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/10/30 18:45, Pingfan Liu wrote:
> On Thu, Oct 30, 2025 at 02:44:43PM +0800, Chen Ridong wrote:
>>
>>
>> On 2025/10/29 19:18, Pingfan Liu wrote:
>>> Hi Ridong,
>>>
>>> Thank you for your review, please see the comment below.
>>>
>>> On Wed, Oct 29, 2025 at 10:37:47AM +0800, Chen Ridong wrote:
>>>>
>>>>
>>>> On 2025/10/28 11:43, Pingfan Liu wrote:
>>>>> *** Bug description ***
>>>>> When testing kexec-reboot on a 144 cpus machine with
>>>>> isolcpus=managed_irq,domain,1-71,73-143 in kernel command line, I
>>>>> encounter the following bug:
>>>>>
>>>>> [   97.114759] psci: CPU142 killed (polled 0 ms)
>>>>> [   97.333236] Failed to offline CPU143 - error=-16
>>>>> [   97.333246] ------------[ cut here ]------------
>>>>> [   97.342682] kernel BUG at kernel/cpu.c:1569!
>>>>> [   97.347049] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>>>>> [...]
>>>>>
>>>>> In essence, the issue originates from the CPU hot-removal process, not
>>>>> limited to kexec. It can be reproduced by writing a SCHED_DEADLINE
>>>>> program that waits indefinitely on a semaphore, spawning multiple
>>>>> instances to ensure some run on CPU 72, and then offlining CPUs 1–143
>>>>> one by one. When attempting this, CPU 143 failed to go offline.
>>>>>   bash -c 'taskset -cp 0 $$ && for i in {1..143}; do echo 0 > /sys/devices/system/cpu/cpu$i/online 2>/dev/null; done'
>>>>>
>>>>> `
>>>>> *** Issue ***
>>>>> Tracking down this issue, I found that dl_bw_deactivate() returned
>>>>> -EBUSY, which caused sched_cpu_deactivate() to fail on the last CPU.
>>>>> But that is not the fact, and contributed by the following factors:
>>>>> When a CPU is inactive, cpu_rq()->rd is set to def_root_domain. For an
>>>>> blocked-state deadline task (in this case, "cppc_fie"), it was not
>>>>> migrated to CPU0, and its task_rq() information is stale. So its rq->rd
>>>>> points to def_root_domain instead of the one shared with CPU0.  As a
>>>>> result, its bandwidth is wrongly accounted into a wrong root domain
>>>>> during domain rebuild.
>>>>>
>>>>> The key point is that root_domain is only tracked through active rq->rd.
>>>>> To avoid using a global data structure to track all root_domains in the
>>>>> system, there should be a method to locate an active CPU within the
>>>>> corresponding root_domain.
>>>>>
>>>>> *** Solution ***
>>>>> To locate the active cpu, the following rules for deadline
>>>>> sub-system is useful
>>>>>   -1.any cpu belongs to a unique root domain at a given time
>>>>>   -2.DL bandwidth checker ensures that the root domain has active cpus.
>>>>>
>>>>> Now, let's examine the blocked-state task P.
>>>>> If P is attached to a cpuset that is a partition root, it is
>>>>> straightforward to find an active CPU.
>>>>> If P is attached to a cpuset that has changed from 'root' to 'member',
>>>>> the active CPUs are grouped into the parent root domain. Naturally, the
>>>>> CPUs' capacity and reserved DL bandwidth are taken into account in the
>>>>> ancestor root domain. (In practice, it may be unsafe to attach P to an
>>>>> arbitrary root domain, since that domain may lack sufficient DL
>>>>> bandwidth for P.) Again, it is straightforward to find an active CPU in
>>>>> the ancestor root domain.
>>>>>
>>>>> This patch groups CPUs into isolated and housekeeping sets. For the
>>>>> housekeeping group, it walks up the cpuset hierarchy to find active CPUs
>>>>> in P's root domain and retrieves the valid rd from cpu_rq(cpu)->rd.
>>>>>
>>>>> Signed-off-by: Pingfan Liu <piliu@redhat.com>
>>>>> Cc: Waiman Long <longman@redhat.com>
>>>>> Cc: Tejun Heo <tj@kernel.org>
>>>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>>>> Cc: "Michal Koutný" <mkoutny@suse.com>
>>>>> Cc: Ingo Molnar <mingo@redhat.com>
>>>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>>>> Cc: Juri Lelli <juri.lelli@redhat.com>
>>>>> Cc: Pierre Gondois <pierre.gondois@arm.com>
>>>>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>>>>> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
>>>>> Cc: Steven Rostedt <rostedt@goodmis.org>
>>>>> Cc: Ben Segall <bsegall@google.com>
>>>>> Cc: Mel Gorman <mgorman@suse.de>
>>>>> Cc: Valentin Schneider <vschneid@redhat.com>
>>>>> To: cgroups@vger.kernel.org
>>>>> To: linux-kernel@vger.kernel.org
>>>>> ---
>>>>> v3 -> v4:
>>>>> rename function with cpuset_ prefix
>>>>> improve commit log
>>>>>
>>>>>  include/linux/cpuset.h  | 18 ++++++++++++++++++
>>>>>  kernel/cgroup/cpuset.c  | 26 ++++++++++++++++++++++++++
>>>>>  kernel/sched/deadline.c | 30 ++++++++++++++++++++++++------
>>>>>  3 files changed, 68 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
>>>>> index 2ddb256187b51..d4da93e51b37b 100644
>>>>> --- a/include/linux/cpuset.h
>>>>> +++ b/include/linux/cpuset.h
>>>>> @@ -12,6 +12,7 @@
>>>>>  #include <linux/sched.h>
>>>>>  #include <linux/sched/topology.h>
>>>>>  #include <linux/sched/task.h>
>>>>> +#include <linux/sched/housekeeping.h>
>>>>>  #include <linux/cpumask.h>
>>>>>  #include <linux/nodemask.h>
>>>>>  #include <linux/mm.h>
>>>>> @@ -130,6 +131,7 @@ extern void rebuild_sched_domains(void);
>>>>>  
>>>>>  extern void cpuset_print_current_mems_allowed(void);
>>>>>  extern void cpuset_reset_sched_domains(void);
>>>>> +extern void cpuset_get_task_effective_cpus(struct task_struct *p, struct cpumask *cpus);
>>>>>  
>>>>>  /*
>>>>>   * read_mems_allowed_begin is required when making decisions involving
>>>>> @@ -276,6 +278,22 @@ static inline void cpuset_reset_sched_domains(void)
>>>>>  	partition_sched_domains(1, NULL, NULL);
>>>>>  }
>>>>>  
>>>>> +static inline void cpuset_get_task_effective_cpus(struct task_struct *p,
>>>>> +		struct cpumask *cpus)
>>>>> +{
>>>>> +	const struct cpumask *hk_msk;
>>>>> +
>>>>> +	hk_msk = housekeeping_cpumask(HK_TYPE_DOMAIN);
>>>>> +	if (housekeeping_enabled(HK_TYPE_DOMAIN)) {
>>>>> +		if (!cpumask_intersects(p->cpus_ptr, hk_msk)) {
>>>>> +			/* isolated cpus belong to a root domain */
>>>>> +			cpumask_andnot(cpus, cpu_active_mask, hk_msk);
>>>>> +			return;
>>>>> +		}
>>>>> +	}
>>>>> +	cpumask_and(cpus, cpu_active_mask, hk_msk);
>>>>> +}
>>>>> +
>>>>>  static inline void cpuset_print_current_mems_allowed(void)
>>>>>  {
>>>>>  }
>>>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>>>> index 27adb04df675d..6ad88018f1a4e 100644
>>>>> --- a/kernel/cgroup/cpuset.c
>>>>> +++ b/kernel/cgroup/cpuset.c
>>>>> @@ -1102,6 +1102,32 @@ void cpuset_reset_sched_domains(void)
>>>>>  	mutex_unlock(&cpuset_mutex);
>>>>>  }
>>>>>  
>>>>> +/* caller hold RCU read lock */
>>>>> +void cpuset_get_task_effective_cpus(struct task_struct *p, struct cpumask *cpus)
>>>>> +{
>>>>> +	const struct cpumask *hk_msk;
>>>>> +	struct cpuset *cs;
>>>>> +
>>>>> +	hk_msk = housekeeping_cpumask(HK_TYPE_DOMAIN);
>>>>> +	if (housekeeping_enabled(HK_TYPE_DOMAIN)) {
>>>>> +		if (!cpumask_intersects(p->cpus_ptr, hk_msk)) {
>>>>> +			/* isolated cpus belong to a root domain */
>>>>> +			cpumask_andnot(cpus, cpu_active_mask, hk_msk);
>>>>> +			return;
>>>>> +		}
>>>>> +	}
>>>>> +	/* In HK_TYPE_DOMAIN, cpuset can be applied */
>>>>> +	cs = task_cs(p);
>>>>> +	while (cs != &top_cpuset) {
>>>>> +		if (is_sched_load_balance(cs))
>>>>> +			break;
>>>>> +		cs = parent_cs(cs);
>>>>> +	}
>>>>> +
>>>>> +	/* For top_cpuset, its effective_cpus does not exclude isolated cpu */
>>>>> +	cpumask_and(cpus, cs->effective_cpus, hk_msk);
>>>>> +}
>>>>> +
>>>>
>>>> It seems you may have misunderstood what Longman intended to convey.
>>>>
>>>
>>> Thanks for pointing that out. That is possible and please let me address
>>> your concern.
>>>
>>>> First, you should add comments to this function because its purpose is not clear. When I first saw
>>>
>>> OK, I will.
>>>
>>>> this function, I thought it was supposed to retrieve p->cpus_ptr excluding the offline CPU mask.
>>>> However, I'm genuinely confused about the function's actual purpose.
>>>>
>>>
>>> This function retrieves the active CPUs within the root domain where a specified task resides.
>>>
>>
>> Thank you for the further clarification.
>>
>> 	+	/*
>> 	+	 * If @p is in blocked state, task_cpu() may be not active. In that
>> 	+	 * case, rq->rd does not trace a correct root_domain. On the other hand,
>> 	+	 * @p must belong to an root_domain at any given time, which must have
>> 	+	 * active rq, whose rq->rd traces the valid root domain.
>> 	+	 */
>>
>> Is it necessary to walk up to the root partition (is_sched_load_balance(cs))?
>>
>> The effective_cpus of the cpuset where @p resides should contain active CPUs.
>> If all CPUs in cpuset.cpus are offline, it would inherit the parent's effective_cpus for v2, and it
>> would move the task to the parent for v1.
>>
> 
> Suppose that the parent cpuset has no active CPUs too.
> But for a root_domain, deadline bandwidth validation can guard there are
> active CPUs remaining.
> 

I don't think this should happen. When a parent's effective_cpus is empty, it should inherit its own
parent's effective_cpus as well, meaning that in v2, the effective_cpus should not ultimately remain
empty.

For v1, the task would be moved to an ancestor with active cpus.

-- 
Best regards,
Ridong