Re: [PATCH next] cpuset: fix warning when attaching tasks with offline CPUs

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 14, 2025 at 03:23:11AM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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@huawei.com>
> Signed-off-by: Chen Ridong <chenridong@huawei.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);

This breaks things. Any task mask must be a subset of
task_cpu_possible_mask() at all times. It might not be able to run
outside of that mask.