Re: [PATCH] sched/deadline: Make dl-server nohz full aware

[Juri Lelli <juri.lelli@redhat.com>]

Hi Andrea,

On 12/05/26 16:55, Andrea Righi wrote:
> Hi Juri,

Thanks from the quick review!

> On Tue, May 12, 2026 at 11:02:37AM +0200, Juri Lelli wrote:
> > The dl_server_timer() causes spurious IPIs on nohz_full cores, breaking
> > isolation guarantees. The timer executes on a housekeeping core and
> > eventually calls tick_nohz_dep_set_cpu(), sending IPIs to isolated cores
> > even when only a single task is running.
> > 
> > The problem is that dl-servers are not coordinated with nohz_full tick
> > state. Timers can fire and send IPIs to otherwise undisturbed cores.
> > 
> > Fix by managing servers in sched_can_stop_tick():
> > 
> > - When RT tasks run with CFS/SCX tasks, start the appropriate server
> >   and keep the tick running
> > - When only RT tasks remain, stop all servers and allow tick to stop
> >   (except for >1 RR tasks which need the tick for round-robin)
> > - When only CFS/SCX tasks remain, stop all servers before stopping tick
> > 
> > Introduce dl_servers_stop_all() to reduce duplication and abstract
> > server management from core.c. Unify RT handling into one block that
> > handles both RR and FIFO cases.
> > 
> > Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
> > Reported-by: David Haufe <dhaufe@simplextrading.com>
> > Closes: https://lore.kernel.org/lkml/CAKJHwtOw_G67edzuHVtL1xC5Vyt6StcZzihtDd0yaKudW=rwVw@mail.gmail.com
> > Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> > ---
> > I had to modify my first original attempt at fixing this (please take a
> > look at the linked report/discussion) to also take SCX into
> > consideration.
> 
> As mentioned by Frederic, we don't allow to load BPF schedulers when isolcpus=
> is used, so I think we can simplify the sched_can_stop_tick() part.

Right! Thanks for confirming.

> > 
> > FYI, I temporarily pushed the script I'm using to repro and verify the
> > fix here
> > 
> > https://github.com/jlelli/sched-deadline-tests/blob/master/test-dlserver-nohz.sh
> > ---
> >  kernel/sched/core.c     | 43 +++++++++++++++++++++++--------------------
> >  kernel/sched/deadline.c | 14 ++++++++++++++
> >  kernel/sched/sched.h    |  1 +
> >  3 files changed, 38 insertions(+), 20 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b905805bbcbe4..98759255c306b 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1414,30 +1414,35 @@ static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
> >  
> >  bool sched_can_stop_tick(struct rq *rq)
> >  {
> > -	int fifo_nr_running;
> > -
> >  	/* Deadline tasks, even if single, need the tick */
> >  	if (rq->dl.dl_nr_running)
> >  		return false;
> >  
> >  	/*
> > -	 * If there are more than one RR tasks, we need the tick to affect the
> > -	 * actual RR behaviour.
> > +	 * If there are RT tasks, we may need the tick (for >1 RR tasks),
> > +	 * but we must also service lower-priority CFS/SCX tasks via dl-servers.
> 
> No need to mention SCX, maybe we can add a note that SCX is incompatible with
> isolcpus, so there's no SCX task to run here.

Ack.

> 
> >  	 */
> > -	if (rq->rt.rr_nr_running) {
> > -		if (rq->rt.rr_nr_running == 1)
> > -			return true;
> > -		else
> > +	if (rq->rt.rt_nr_running) {
> > +		if (rq->cfs.h_nr_queued) {
> > +			dl_server_start(&rq->fair_server);
> > +			return false;
> > +		}
> > +#ifdef CONFIG_SCHED_CLASS_EXT
> > +		if (rq->scx.nr_running) {
> > +			dl_server_start(&rq->ext_server);
> > +			return false;
> > +		}
> > +#endif
> 
> This #ifdef block can go away.
> 
> > +		/*
> > +		 * Only RT tasks, no CFS/SCX. Stop servers to prevent spurious
> 
> CFS/SCX -> CFS.
> 
> > +		 * wakeups. Tick can stop for single RR or any FIFO, but must
> > +		 * run for multiple RR (round-robin behavior).
> > +		 */
> > +		dl_servers_stop_all(rq);
> > +		if (rq->rt.rr_nr_running > 1)
> >  			return false;
> > -	}
> > -
> > -	/*
> > -	 * If there's no RR tasks, but FIFO tasks, we can skip the tick, no
> > -	 * forced preemption between FIFO tasks.
> > -	 */
> > -	fifo_nr_running = rq->rt.rt_nr_running - rq->rt.rr_nr_running;
> > -	if (fifo_nr_running)
> >  		return true;
> > +	}
> >  
> >  	/*
> >  	 * If there are no DL,RR/FIFO tasks, there must only be CFS or SCX tasks
> > @@ -1462,6 +1467,7 @@ bool sched_can_stop_tick(struct rq *rq)
> >  			return false;
> >  	}
> >  
> > +	dl_servers_stop_all(rq);
> >  	return true;
> >  }
> >  #endif /* CONFIG_NO_HZ_FULL */
> > @@ -8810,10 +8816,7 @@ int sched_cpu_dying(unsigned int cpu)
> >  		WARN(true, "Dying CPU not properly vacated!");
> >  		dump_rq_tasks(rq, KERN_WARNING);
> >  	}
> > -	dl_server_stop(&rq->fair_server);
> > -#ifdef CONFIG_SCHED_CLASS_EXT
> > -	dl_server_stop(&rq->ext_server);
> > -#endif
> > +	dl_servers_stop_all(rq);
> >  	rq_unlock_irqrestore(rq, &rf);
> >  
> >  	calc_load_migrate(rq);
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index edca7849b165d..c2b3d6bbe4828 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1826,6 +1826,20 @@ void dl_server_stop(struct sched_dl_entity *dl_se)
> >  	dl_se->dl_server_active = 0;
> >  }
> >  
> > +/*
> > + * Stop all dl-servers on this runqueue. Called when transitioning to a state
> > + * where the tick can be stopped (e.g., single RR/FIFO task, or no RT tasks).
> > + * This ensures server timers are disarmed and won't cause spurious wakeups on
> > + * nohz_full isolated cores.
> > + */
> > +void dl_servers_stop_all(struct rq *rq)
> > +{
> > +	dl_server_stop(&rq->fair_server);
> > +#ifdef CONFIG_SCHED_CLASS_EXT
> > +	dl_server_stop(&rq->ext_server);
> > +#endif
> > +}
> 
> And I think the dl_servers_stop_all() helper still makes sense, stopping the
> ext_server is still needed in sched_cpu_dying() and calling dl_server_stop() on
> an already-inactive server is harmless in the no-RT path.

And ack to all the above. Will send out a v2 soon.

Best,
Juri