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

[Juri Lelli]

On 12/05/26 16:03, Frederic Weisbecker wrote:
> 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...

So, the IPIs seem to be indeed gone also from my recent testing (decided
to keep the reference/context in the changelog for historical reasons,
but can remove). But the dl_task_timer (dl-server timer) for isolated
CPUs is still firing on housekeeping and that is what this is
addressing.

> > ---
> > 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")

Ah, thanks for poiting it out, it indeed simplify things. :)

And thanks for the super quick review!

Best,
Juri