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

[Pingfan Liu <piliu@redhat.com>]

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.

> 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?
> 

Yes, that is the purpose.

Best Regards,

Pingfan