Re: [PATCH 06/11] sched_ext: Add a DL server for sched_ext tasks

Juri Lelli
2025-11-06T11:00:02Z

Hi,

On 29/10/25 20:08, Andrea Righi wrote:
> sched_ext currently suffers starvation due to RT. The same workload when
> converted to EXT can get zero runtime if RT is 100% running, causing EXT
> processes to stall. Fix it by adding a DL server for EXT.
>
> A kselftest is also provided later to verify:
>
> # ./runner -t rt_stall
> ===== START =====
> TEST: rt_stall
> DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> OUTPUT:
> # Runtime of EXT task (PID 23338) is 0.250000 seconds
> # Runtime of RT task (PID 23339) is 4.750000 seconds
> # EXT task got 5.00% of total runtime
> ok 1 PASS: EXT task got more than 4.00% of runtime
> ===== END =====
>
> v3: - clarify that fair is not the only dl_server (Juri Lelli)
> - remove explicit stop to reduce timer reprogramming overhead
> (Juri Lelli)
> - do not restart pick_task() when it's invoked by the dl_server
> (Tejun Heo)
> - depend on CONFIG_SCHED_CLASS_EXT (Andrea Righi)
> v2: - drop ->balance() now that pick_task() has an rf argument
> (Andrea Righi)
>
> Cc: Luigi De Matteis <ldematteis123@gmail.com>
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---

...

> @@ -1409,6 +1412,15 @@ 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);
>
> + if (rq->scx.nr_running == 1) {
> + /* Account for idle runtime */
> + if (!rq->nr_running)

Hummm, didn't we just add_nr_running(rq, 1) before gettng here?

> + dl_server_update_idle_time(rq, rq->curr, &rq->ext_server);
> +
> + /* Start dl_server if this is the first task being enqueued */
> + dl_server_start(&rq->ext_server);
> + }
> +

Thanks,
Juri