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Fri, 31 Oct 2025 10:21:23 -0400 X-MC-Unique: icmdw5hsPMyWMuzkV_juzg-1 X-Mimecast-MFC-AGG-ID: icmdw5hsPMyWMuzkV_juzg_1761920481 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 311521954B06; Fri, 31 Oct 2025 14:21:20 +0000 (UTC) Received: from localhost (unknown [10.72.112.72]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 2A3BA19560A2; Fri, 31 Oct 2025 14:21:16 +0000 (UTC) Date: Fri, 31 Oct 2025 22:21:10 +0800 From: Pingfan Liu To: Chen Ridong Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Waiman Long , Peter Zijlstra , Juri Lelli , Pierre Gondois , Frederic Weisbecker , Ingo Molnar , Tejun Heo , Johannes Weiner , Michal =?iso-8859-1?Q?Koutn=FD?= , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider Subject: Re: [PATCHv4 2/2] sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug Message-ID: References: <20251028034357.11055-1-piliu@redhat.com> <20251028034357.11055-2-piliu@redhat.com> <73663a65-8028-4294-8eaf-9c94dc4451ff@huaweicloud.com> <44130515-725a-4f44-b064-3b396ed26159@huaweicloud.com> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On Fri, Oct 31, 2025 at 08:47:14AM +0800, Chen Ridong wrote: > > > 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 > >>>>> Cc: Waiman Long > >>>>> Cc: Tejun Heo > >>>>> Cc: Johannes Weiner > >>>>> Cc: "Michal Koutný" > >>>>> Cc: Ingo Molnar > >>>>> Cc: Peter Zijlstra > >>>>> Cc: Juri Lelli > >>>>> Cc: Pierre Gondois > >>>>> Cc: Vincent Guittot > >>>>> Cc: Dietmar Eggemann > >>>>> Cc: Steven Rostedt > >>>>> Cc: Ben Segall > >>>>> Cc: Mel Gorman > >>>>> Cc: Valentin Schneider > >>>>> 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 > >>>>> #include > >>>>> #include > >>>>> +#include > >>>>> #include > >>>>> #include > >>>>> #include > >>>>> @@ -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. > >> I located the code which implemented your comment. And I think for v2, you are right. But for v1, there is an async nuance about remove_tasks_in_empty_cpuset(). It is scheduled with a work_struct, so there is no gurantee that task has been moved to ancestor cpuset before rebuild_sched_domains_cpuslocked() is called in cpuset_handle_hotplug(), which means that in dl_update_tasks_root_domain(), maybe task's cpuset has not been updated yet. > > > > 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. > You are right. I found the propagating in cpuset_for_each_descendant_pre(). Thanks for your insight. It makes thing more clear! Best Regards, Pingfan