From: Pingfan Liu Date: Wed, 19 Nov 2025 09:55:25 +0000 (+0800) Subject: sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug X-Git-Tag: v6.19-rc1~178^2~1 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=318e18ed22e89397635e15095c014accaf47ed30;p=thirdparty%2Fkernel%2Flinux.git sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug *** 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' 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. *** Issue *** 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: Chen Ridong Cc: Peter Zijlstra Cc: Juri Lelli Cc: Pierre Gondois Cc: Ingo Molnar Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Steven Rostedt Cc: Ben Segall Cc: Mel Gorman Cc: Valentin Schneider To: linux-kernel@vger.kernel.org Signed-off-by: Tejun Heo --- diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 7b7671060bf9e..194a341e85864 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -2465,6 +2465,7 @@ static struct task_struct *pick_earliest_pushable_dl_task(struct rq *rq, int cpu return NULL; } +/* Access rule: must be called on local CPU with preemption disabled */ static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl); static int find_later_rq(struct task_struct *task) @@ -2907,11 +2908,43 @@ void __init init_sched_dl_class(void) GFP_KERNEL, cpu_to_node(i)); } +/* + * This function always returns a non-empty bitmap in @cpus. This is because + * if a root domain has reserved bandwidth for DL tasks, the DL bandwidth + * check will prevent CPU hotplug from deactivating all CPUs in that domain. + */ +static void dl_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)) { + /* + * CPUs isolated by isolcpu="domain" always belong to + * def_root_domain. + */ + cpumask_andnot(cpus, cpu_active_mask, hk_msk); + return; + } + } + + /* + * If a root domain holds a DL task, it must have active CPUs. So + * active CPUs can always be found by walking up the task's cpuset + * hierarchy up to the partition root. + */ + cpuset_cpus_allowed_locked(p, cpus); +} + +/* The caller should hold cpuset_mutex */ void dl_add_task_root_domain(struct task_struct *p) { struct rq_flags rf; struct rq *rq; struct dl_bw *dl_b; + unsigned int cpu; + struct cpumask *msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl); raw_spin_lock_irqsave(&p->pi_lock, rf.flags); if (!dl_task(p) || dl_entity_is_special(&p->dl)) { @@ -2919,16 +2952,25 @@ void dl_add_task_root_domain(struct task_struct *p) return; } - rq = __task_rq_lock(p, &rf); - + /* + * Get an active rq, whose rq->rd traces the correct root + * domain. + * Ideally this would be under cpuset reader lock until rq->rd is + * fetched. However, sleepable locks cannot nest inside pi_lock, so we + * rely on the caller of dl_add_task_root_domain() holds 'cpuset_mutex' + * to guarantee the CPU stays in the cpuset. + */ + dl_get_task_effective_cpus(p, msk); + cpu = cpumask_first_and(cpu_active_mask, msk); + BUG_ON(cpu >= nr_cpu_ids); + rq = cpu_rq(cpu); dl_b = &rq->rd->dl_bw; - raw_spin_lock(&dl_b->lock); + /* End of fetching rd */ + raw_spin_lock(&dl_b->lock); __dl_add(dl_b, p->dl.dl_bw, cpumask_weight(rq->rd->span)); - raw_spin_unlock(&dl_b->lock); - - task_rq_unlock(rq, p, &rf); + raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags); } void dl_clear_root_domain(struct root_domain *rd)