Message-ID: <44130515-725a-4f44-b064-3b396ed26159@huaweicloud.com>
Date: Thu, 30 Oct 2025 14:44:43 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Pingfan Liu <piliu@...hat.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 Waiman Long <longman@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>, Pierre Gondois <pierre.gondois@....com>,
 Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCHv4 2/2] sched/deadline: Walk up cpuset hierarchy to decide
 root domain when hot-unplug



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@...hat.com>
>>> Cc: Waiman Long <longman@...hat.com>
>>> Cc: Tejun Heo <tj@...nel.org>
>>> Cc: Johannes Weiner <hannes@...xchg.org>
>>> Cc: "Michal Koutný" <mkoutny@...e.com>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Juri Lelli <juri.lelli@...hat.com>
>>> Cc: Pierre Gondois <pierre.gondois@....com>
>>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>>> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
>>> Cc: Steven Rostedt <rostedt@...dmis.org>
>>> Cc: Ben Segall <bsegall@...gle.com>
>>> Cc: Mel Gorman <mgorman@...e.de>
>>> Cc: Valentin Schneider <vschneid@...hat.com>
>>> To: cgroups@...r.kernel.org
>>> To: linux-kernel@...r.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.

Could the effective_cpus of @p's current cpuset be sufficient?
What we really need is to find active CPUs that task P can be affine to, correct?

Hope I didn't miss something.

-- 
Best regards,
Ridong

Powered by blists - more mailing lists