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

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Message-Id: <20250714032311.3570157-1-chenridong@huaweicloud.com>
Date: Mon, 14 Jul 2025 03:23:11 +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 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.

The fix changes cpus_attach initialization to use cpu_active_mask
instead of cpu_possible_mask, ensuring we only consider online CPUs
when setting the new affinity. This prevents the scenario where
the intersection would result in an invalid CPU set.

Fixes: da019032819a ("sched: Enforce user requested affinity")
Reported-by: Yang Lijin <yanglijin@...wei.com>
Signed-off-by: Chen Ridong <chenridong@...wei.com>
---
 kernel/cgroup/cpuset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index f74d04429a29..5401adbdffa6 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3121,7 +3121,7 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 	if (cs != &top_cpuset)
 		guarantee_active_cpus(task, cpus_attach);
 	else
-		cpumask_andnot(cpus_attach, task_cpu_possible_mask(task),
+		cpumask_andnot(cpus_attach, cpu_active_mask,
 			       subpartitions_cpus);
 	/*
 	 * can_attach beforehand should guarantee that this doesn't
-- 
2.34.1