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

[Frederic Weisbecker]

Le Tue, May 12, 2026 at 11:02:37AM +0200, Juri Lelli a écrit :
> 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 indeed observed IPIs originating from dl_server some time
ago but that magically disappeared after some commit from Peter.

Perhaps it came back somehow? Lemme run dynticks-testing again on
latest upstream...

> ---
> 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.

I thought SCX was disabled when CPU isolation is running?

9f391f94a173 ("sched_ext: Disallow loading BPF scheduler if isolcpus= domain
isolation is in effect")

Thanks.

-- 
Frederic Weisbecker
SUSE Labs