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

[Furkan Çalışkan]

Hi Juri,

On 5/12/26 12:02, 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@xxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/CAKJHwtOw_G67edzuHVtL1xC5Vyt6StcZzihtDd0yaKudW=rwVw@xxxxxxxxxxxxxx
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> ---
> 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.
> 
> 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.
>  	 */
> -	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

In the above block, the CFS and SCX server start paths are mutually exclusive. 
If both cfs.h_nr_queued and scx.nr_running are non-zero at the same time, only 
fair_server gets started and ext_server remains stopped. Could that leave SCX 
tasks without server coverage in a mixed CFS+SCX+RT scenario?

---
Thanks,
Furkan Caliskan