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Thu, 30 Oct 2025 06:45:40 -0400 X-MC-Unique: lpjtSDpaPhKlUf6zK_dj1w-1 X-Mimecast-MFC-AGG-ID: lpjtSDpaPhKlUf6zK_dj1w_1761821138 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2447D195606F; Thu, 30 Oct 2025 10:45:38 +0000 (UTC) Received: from localhost (unknown [10.72.112.70]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B1CB219560B7; Thu, 30 Oct 2025 10:45:34 +0000 (UTC) Date: Thu, 30 Oct 2025 18:45:29 +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: <44130515-725a-4f44-b064-3b396ed26159@huaweicloud.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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. > 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