linux-kernel - [PATCH v2 -next] cpuset: fix warning when attaching tasks with offline CPUs

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Message-Id: <20250715023340.3617147-1-chenridong@huaweicloud.com>
Date: Tue, 15 Jul 2025 02:33:40 +0000
From: Chen Ridong <chenridong@...weicloud.com>
To: longman@...hat.com,
	tj@...nel.org,
	hannes@...xchg.org,
	mkoutny@...e.com,
	peterz@...radead.org
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	lujialin4@...wei.com,
	chenridong@...wei.com
Subject: [PATCH v2 -next] cpuset: fix warning when attaching tasks with offline CPUs

From: Chen Ridong <chenridong@...wei.com>

A kernel warning was observed in the cpuset migration path:

     WARNING: CPU: 3 PID: 123 at kernel/cgroup/cpuset.c:3130
     cgroup_migrate_execute+0x8df/0xf30
     Call Trace:
      cgroup_transfer_tasks+0x2f3/0x3b0
      cpuset_migrate_tasks_workfn+0x146/0x3b0
      process_one_work+0x5ba/0xda0
      worker_thread+0x788/0x1220

The issue can be reliably reproduced with:

     # Setup test cpuset
     mkdir /sys/fs/cgroup/cpuset/test
     echo 2-3 > /sys/fs/cgroup/cpuset/test/cpuset.cpus
     echo 0 > /sys/fs/cgroup/cpuset/test/cpuset.mems

     # Start test process
     sleep 100 &
     pid=$!
     echo $pid > /sys/fs/cgroup/cpuset/test/cgroup.procs
     taskset -p 0xC $pid  # Bind to CPUs 2-3

     # Take CPUs offline
     echo 0 > /sys/devices/system/cpu/cpu3/online
     echo 0 > /sys/devices/system/cpu/cpu2/online

Root cause analysis:
When tasks are migrated to top_cpuset due to CPUs going offline,
cpuset_attach_task() sets the CPU affinity using cpus_attach which
is initialized from cpu_possible_mask. This mask may include offline
CPUs. When __set_cpus_allowed_ptr() computes the intersection between:
1. cpus_attach (possible CPUs, may include offline)
2. p->user_cpus_ptr (original user-set mask)
The resulting new_mask may contain only offline CPUs, causing the
operation to fail.

To resolve this issue, if the call to set_cpus_allowed_ptr fails, retry
using the intersection of cpus_attach and cpu_active_mask.

Fixes: da019032819a ("sched: Enforce user requested affinity")
Suggested-by: Waiman Long <llong@...hat.com>
Reported-by: Yang Lijin <yanglijin@...wei.com>
Signed-off-by: Chen Ridong <chenridong@...wei.com>
---
 kernel/cgroup/cpuset.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index f74d04429a29..2cf788a8982a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3114,6 +3114,10 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 static cpumask_var_t cpus_attach;
 static nodemask_t cpuset_attach_nodemask_to;
 
+/*
+ * Note that tasks in the top cpuset won't get update to their cpumasks when
+ * a hotplug event happens. So we include offline CPUs as well.
+ */
 static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 {
 	lockdep_assert_held(&cpuset_mutex);
@@ -3127,7 +3131,16 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 	 * can_attach beforehand should guarantee that this doesn't
 	 * fail.  TODO: have a better way to handle failure here
 	 */
-	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
+	if (unlikely(set_cpus_allowed_ptr(task, cpus_attach))) {
+		/*
+		 * Since offline CPUs are included for top_cpuset,
+		 * set_cpus_allowed_ptr() can fail if user_cpus_ptr contains
+		 * only offline CPUs. Take out the offline CPUs and retry.
+		 */
+		if (cs == &top_cpuset)
+			cpumask_and(cpus_attach, cpus_attach, cpu_active_mask);
+		WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
+	}
 
 	cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
 	cpuset1_update_task_spread_flags(cs, task);
-- 
2.34.1