Re: [PATCH 4/7] sched_ext: Add a DL server for sched_ext tasks

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 02, 2026 at 09:32:32PM +0100, Andrea Righi wrote:
> Hi Peter,
> 
> On Mon, Feb 02, 2026 at 08:50:35PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 26, 2026 at 10:59:02AM +0100, Andrea Righi wrote:
> > 
> > > @@ -3181,6 +3193,36 @@ void dl_add_task_root_domain(struct task_struct *p)
> > >  	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
> > >  }
> > >  
> > > +static void dl_server_add_bw(struct root_domain *rd, int cpu)
> > > +{
> > > +	struct sched_dl_entity *dl_se;
> > > +
> > > +	dl_se = &cpu_rq(cpu)->fair_server;
> > > +	if (dl_server(dl_se) && cpu_active(cpu))
> > > +		__dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(cpu));
> > > +
> > > +#ifdef CONFIG_SCHED_CLASS_EXT
> > > +	dl_se = &cpu_rq(cpu)->ext_server;
> > > +	if (dl_server(dl_se) && cpu_active(cpu))
> > > +		__dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(cpu));
> > > +#endif
> > > +}
> > > +
> > > +static u64 dl_server_read_bw(int cpu)
> > > +{
> > > +	u64 dl_bw = 0;
> > > +
> > > +	if (cpu_rq(cpu)->fair_server.dl_server)
> > > +		dl_bw += cpu_rq(cpu)->fair_server.dl_bw;
> > > +
> > > +#ifdef CONFIG_SCHED_CLASS_EXT
> > > +	if (cpu_rq(cpu)->ext_server.dl_server)
> > > +		dl_bw += cpu_rq(cpu)->ext_server.dl_bw;
> > > +#endif
> > > +
> > > +	return dl_bw;
> > > +}
> > 
> > Should not this also depend on scx_enabled()? It seems unfortunate to
> > consume bandwidth if scx isn't even enabled.
> 
> Yeah, that's a good point. We can just add scx_enabled() here. Let me try
> running some tests with this.

I suspect you need some callbacks around where scx_enabled() is changed
to add/stop/remove things.

> > 
> > > @@ -1501,6 +1503,10 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags
> > >  	if (enq_flags & SCX_ENQ_WAKEUP)
> > >  		touch_core_sched(rq, p);
> > >  
> > > +	/* Start dl_server if this is the first task being enqueued */
> > > +	if (rq->scx.nr_running == 1)
> > > +		dl_server_start(&rq->ext_server);
> > > +
> > >  	do_enqueue_task(rq, p, enq_flags, sticky_cpu);
> > >  out:
> > >  	rq->scx.flags &= ~SCX_RQ_IN_WAKEUP;
> > 
> > So this starts the dl_server for the CPU the thing gets enqueued on, but
> > SCX being what it is, there is absolutely no guarantee its actually ever
> > pickable from there, right?
> > 
> > Does it make sense to delay this until its a DSQ_LOCAL enqueue? Or will
> > it never get that far without help?
> 
> We could probably move dl_server_start() to local_dsq_post_enq() and start
> the dl server when a task is actually dispatched to the local DSQ. I think
> it should work, but I'd like to do more extensive testing with this change.
> 
> In any case, IMHO these are improvements we can make later, even in its
> current form, this is still better than what we have and it would at least
> address the recurring bug reports about RT tasks starving scx tasks...

Yeah, we can do on top.