Re: [RFC v3 1/2] workqueue: Add support for real-time workers

[Tejun Heo <tj@kernel.org>]

Hello, sorry about the delay.

On Fri, Jul 17, 2026 at 11:06:55AM +0100, Tvrtko Ursulin wrote:
> > +	/*
> > +	 * Do not consume all CPUs with RT workers to avoid scheduler
> > +	 * starvation.
> > +	 */
> > +	if (pool->attrs->prio == WQ_PRIO_RT) {
> > +		unsigned int max = num_online_cpus();
> > +
> > +		if (max > 2)
> > +			max = max - 1;
> > +		else
> > +			max = 1;
> > +
> > +		if (atomic_inc_return(&total_rtpri_workers) > max) {
> > +			atomic_dec(&total_rtpri_workers);
> > +			return NULL;
> 
> Sashiko is warning me here this can create a deadlock when called from
> maybe_create_worker(). My apparently naive idea was that this would make it
> wait for a bit, until the load on the currently instantiated workers
> subsides, at which point need_to_create_worker() would say "not any more"
> and allow it to exit.
> 
> But even more importantly, it is also warning me that strict CPU affinity
> creates per CPU threads.
> 
> So my idea of global limit will just not work.
> 
> Tejun, how does this sound to you? Are you okay without having the global
> limit of RT threads?
> 
> I will await sending v4 with other fixes until we can close on this one. I
> also have some vacation pending so there may be a delay in my replies.

I don't think we need to limit the total number of workers. It's not a
meaningful protection anyway (is it ok if max-1 CPUs are pinned in RT?), the
DL server already provides forward progress protection, and it doesn't make
sense to design this assuming malicious / incompetent users.

Thanks.

-- 
tejun