Re: [PATCH 2/5] cgroup/cpuset: Include offline CPUs when tasks' cpumasks in top_cpuset are updated

Hello Waiman.

On Mon, Mar 06, 2023 at 03:08:46PM -0500, Waiman Long longman@redhat.com wrote:

• /*
  

•  * Percpu kthreads in top_cpuset are ignored
  

•  */
  

• if (top_cs && (task->flags & PF_KTHREAD) &&
  

•     kthread_is_per_cpu(task))
  

• 	continue;
  

• const struct cpumask *possible_mask = task_cpu_possible_mask(task);
  

• cpumask_and(new_cpus, cs->effective_cpus,
  

• 	    task_cpu_possible_mask(task));
  

• if (top_cs) {
  

• 	/*
  

• 	 * Percpu kthreads in top_cpuset are ignored
  

• 	 */
  

• 	if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task))
  

• 		continue;
  

• 	cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus);
  

• } else {
  

• 	cpumask_and(new_cpus, cs->effective_cpus, possible_mask);
  

• }
  

I'm wrapping my head around this slightly. 1) I'd suggest swapping args in of cpumask_and() to have possible_mask consistently first. 2) Then I'm wondering whether two branches are truly different when effective_cpus := cpus_allowed - subparts_cpus top_cpuset.cpus_allowed == possible_mask (1)

IOW, can you see a difference in what affinities are set to eligible top_cpuset tasks before and after this patch upon CPU hotplug? (Hm, (1) holds only in v2. So is this a fix for v1 only?)

Thanks, Michal