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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 20, 2026 at 10:50:35PM +0100, 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 included later to confirm that both DL servers are
> functioning correctly:
> 
>  # ./runner -t rt_stall
>  ===== START =====
>  TEST: rt_stall
>  DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
>  OUTPUT:
>  TAP version 13
>  1..1
>  # Runtime of FAIR task (PID 1511) is 0.250000 seconds
>  # Runtime of RT task (PID 1512) is 4.750000 seconds
>  # FAIR task got 5.00% of total runtime
>  ok 1 PASS: FAIR task got more than 4.00% of runtime
>  TAP version 13
>  1..1
>  # Runtime of EXT task (PID 1514) is 0.250000 seconds
>  # Runtime of RT task (PID 1515) is 4.750000 seconds
>  # EXT task got 5.00% of total runtime
>  ok 2 PASS: EXT task got more than 4.00% of runtime
>  TAP version 13
>  1..1
>  # Runtime of FAIR task (PID 1517) is 0.250000 seconds
>  # Runtime of RT task (PID 1518) is 4.750000 seconds
>  # FAIR task got 5.00% of total runtime
>  ok 3 PASS: FAIR task got more than 4.00% of runtime
>  TAP version 13
>  1..1
>  # Runtime of EXT task (PID 1521) is 0.250000 seconds
>  # Runtime of RT task (PID 1522) is 4.750000 seconds
>  # EXT task got 5.00% of total runtime
>  ok 4 PASS: EXT task got more than 4.00% of runtime
>  ok 1 rt_stall #
>  =====  END  =====
> 
> v5: - do not restart the EXT server on switch_class() (Juri Lelli)
> v4: - initialize EXT server bandwidth reservation at init time and
>       always keep it active (Andrea Righi)
>     - check for rq->nr_running == 1 to determine when to account idle
>       time (Juri Lelli)
> 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)

FWIW (for all these patches), those v# comments, they go...

> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
> Tested-by: Christian Loehle <christian.loehle@arm.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>
> ---

... here, after the ---.