Re: [PATCH v2] sched/deadline: Reset dl_server execution state on stop

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 30, 2026 at 01:26:20PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 30, 2026 at 01:24:13PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 28, 2026 at 02:41:40PM +0100, Andrea Righi wrote:
> > 
> > > Just to make sure we're testing the same thing, I'm currently using
> > > https://git.kernel.org/pub/scm/linux/kernel/git/arighi/linux.git, branch
> > > scx-dl-server.
> > > 
> > > I'm running this test inside virtme-ng:
> > >   $ vng -vb --config tools/testing/selftests/sched_ext/config
> > >   $ vng -v -- tools/testing/selftests/sched_ext/runner -t rt_stall
> > 
> > Apparently you also have to actually have that runner thing built from
> > that tree.
> > 
> > Anyway, all I seem to be able to get (on x86) is PASS: 1 :/
> 
> Argh, that tree has the dodgy 'fix' in. Let me go revert that.

This seems to work?

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 80c9559a3e30..aa3da4d3b8e3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1036,6 +1036,12 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
 			return;
 		}
 
+		/*
+		 * When [4] D->A is followed by [1] A->B, dl_defer_running
+		 * needs to be cleared, otherwise it will fail to properly
+		 * start the zero-laxity timer.
+		 */
+		dl_se->dl_defer_running = 0;
 		replenish_dl_new_period(dl_se, rq);
 	} else if (dl_server(dl_se) && dl_se->dl_defer) {
 		/*
@@ -1654,6 +1660,12 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
+ *       if (dl_time_before() || dl_entity_overflow)
+ *         dl_defer_running = 0;
+ *         replenish_dl_new_period();
+ *           // fwd period
+ *           dl_throttled = 1;
+ *           dl_defer_armed = 1;
  *       if (!dl_defer_running)
  *         dl_defer_armed = 1;
  *         dl_throttled = 1;