sched/deadline: Fix dl_server stop condition

Gabriel reported that the dl_server doesn't stop as expected.

The problem was found to be the fact that idle time and fair runtime are
treated equally. Both will count towards dl_server runtime and push the
activation forwards when it is in the zero-laxity wait state.

Notably:

  dl_server_update_idle()
    update_curr_dl_se()
      if (dl_defer && dl_throttled && dl_runtime_exceeded())
        hrtimer_try_to_cancel(); // stop timer
	replenish_dl_new_period()
	  deadline = now + dl_deadline; // fwd period
	  runtime = dl_runtime;
        start_dl_timer(); // restart timer

And while we do want idle time accounted towards the *current* activation of
the dl_server -- after all, a fair task could've ran if we had any -- we don't
necessarily want idle time to cause or push forward an activation.

Introduce dl_defer_idle to make this distinction. It will be set once idle time
pushed the activation forward, once set idle time will only be allowed to
consume any runtime but not push the activation. This will then cause
dl_server_timer() to fire, which will stop the dl_server.

Any non-idle time accounting during this phase will clear dl_defer_idle, so
only a full period of idle will cause the dl_server to stop.

Reported-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251101000057.GA2184199@noisy.programming.kicks-ass.net

1 files changed, 9 insertions, 6 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 07576479c0ed..bb436ee1942d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -685,20 +685,22 @@ struct sched_dl_entity {
 	 *
 	 * @dl_server tells if this is a server entity.
 	 *
-	 * @dl_defer tells if this is a deferred or regular server. For
-	 * now only defer server exists.
-	 *
-	 * @dl_defer_armed tells if the deferrable server is waiting
-	 * for the replenishment timer to activate it.
-	 *
 	 * @dl_server_active tells if the dlserver is active(started).
 	 * dlserver is started on first cfs enqueue on an idle runqueue
 	 * and is stopped when a dequeue results in 0 cfs tasks on the
 	 * runqueue. In other words, dlserver is active only when cpu's
 	 * runqueue has atleast one cfs task.
 	 *
+	 * @dl_defer tells if this is a deferred or regular server. For
+	 * now only defer server exists.
+	 *
+	 * @dl_defer_armed tells if the deferrable server is waiting
+	 * for the replenishment timer to activate it.
+	 *
 	 * @dl_defer_running tells if the deferrable server is actually
 	 * running, skipping the defer phase.
+	 *
+	 * @dl_defer_idle tracks idle state
 	 */
 	unsigned int			dl_throttled      : 1;
 	unsigned int			dl_yielded        : 1;
@@ -709,6 +711,7 @@ struct sched_dl_entity {
 	unsigned int			dl_defer	  : 1;
 	unsigned int			dl_defer_armed	  : 1;
 	unsigned int			dl_defer_running  : 1;
+	unsigned int			dl_defer_idle     : 1;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its