On Thu, Nov 06, 2025 at 11:59:56AM +0100, Juri Lelli wrote: > 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 <[email protected]> > > Co-developed-by: Joel Fernandes <[email protected]> > > Signed-off-by: Joel Fernandes <[email protected]> > > Signed-off-by: Andrea Righi <[email protected]> > > --- > > ... > > > @@ -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?
Oh, good catch, let me run some tests to see what happens here. :) But looking at the code, it seems that we definitely need to move add_nr_running() after this part. Thanks! -Andrea > > > + 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 >
