Re: [PATCH/for-next v3 2/3] cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to workqueue

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 02, 2026 at 03:32:03PM -0500, Waiman Long wrote:
> On 2/2/26 3:18 PM, Peter Zijlstra wrote:
> > On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:
> > 
> > > @@ -1310,14 +1321,34 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
> > >    */
> > >   static void update_isolation_cpumasks(void)
> > >   {
> > > -	int ret;
> > > +	static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
> > >   	if (!isolated_cpus_updating)
> > >   		return;
> > > -	ret = housekeeping_update(isolated_cpus);
> > > -	WARN_ON_ONCE(ret < 0);
> > > +	/*
> > > +	 * This function can be reached either directly from regular cpuset
> > > +	 * control file write or via CPU hotplug. In the latter case, it is
> > > +	 * the per-cpu kthread that calls cpuset_handle_hotplug() on behalf
> > > +	 * of the task that initiates CPU shutdown or bringup.
> > > +	 *
> > > +	 * To have better flexibility and prevent the possibility of deadlock
> > > +	 * when calling from CPU hotplug, we defer the housekeeping_update()
> > > +	 * call to after the current cpuset critical section has finished.
> > > +	 * This is done via workqueue.
> > > +	 */
> > > +	if (current->flags & PF_KTHREAD) {
> > 		/* Serializes the static isolcpus_workfn. */
> > 		lockdep_assert_held(&cpuset_mutex);
> 
> Do we require synchronization between the the queue_work() call and the
> execution of the work function? I thought it is not needed, but I may be
> wrong.

Well, something needs to ensure there aren't two threads trying to use
this one work thing at the same time, no?