[RFC PATCH v7 01/29] sched/deadline: Introduce dl_freq_invariant flag for fair/ext servers

[Yuri Andriaccio]

The fair/ext servers are by design frequency and capacity invariant, i.e. their
maximum allowed runtime is fixed, regardless of the frequency the CPU is
running. (See also https://lore.kernel.org/all/20250702021440.2594736-1-kuyo.chang@xxxxxxxxxxxx/)

This inevitably broke non-fair dl_servers, which would want to instead scale
their execution time to be consistent with CBS and GRUB-PA.

The newly added flag allows deadline servers to adopt the frequency invariance
behaviour as fair/ext servers.

Signed-off-by: Yuri Andriaccio <yurand2000@xxxxxxxxx>
---
 include/linux/sched.h   | 4 ++++
 kernel/sched/deadline.c | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1c93da75226e..b78ca828f694 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -706,6 +706,9 @@ struct sched_dl_entity {
 	 * contributes to the root domain's total_bw. Only meaningful for server
 	 * entities (@dl_server == 1). Allows toggling the reservation on/off
 	 * without losing the configured @dl_runtime/@dl_period.
+	 *
+	 * @dl_freq_invariant tells if this server's runtime must NOT be scaled
+	 * by the CPU's frequency and capacity.
 	 */
 	unsigned int			dl_throttled      : 1;
 	unsigned int			dl_yielded        : 1;
@@ -718,6 +721,7 @@ struct sched_dl_entity {
 	unsigned int			dl_defer_running  : 1;
 	unsigned int			dl_defer_idle     : 1;
 	unsigned int			dl_bw_attached    : 1;
+	unsigned int			dl_freq_invariant : 1;

 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index c0ebdcde5fe5..57c7a8802d8e 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1432,7 +1432,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
 		return;

 	scaled_delta_exec = delta_exec;
-	if (!dl_server(dl_se))
+	if (!dl_server(dl_se) || !dl_se->dl_freq_invariant)
 		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);

 	dl_se->runtime -= scaled_delta_exec;
@@ -1862,6 +1862,7 @@ void sched_init_dl_servers(void)

 		dl_se->dl_server = 1;
 		dl_se->dl_defer = 1;
+		dl_se->dl_freq_invariant = 1;
 		setup_new_dl_entity(dl_se);

 #ifdef CONFIG_SCHED_CLASS_EXT
@@ -1873,6 +1874,7 @@ void sched_init_dl_servers(void)

 		dl_se->dl_server = 1;
 		dl_se->dl_defer = 1;
+		dl_se->dl_freq_invariant = 1;
 		setup_new_dl_entity(dl_se);

 		/*
@@ -3957,6 +3959,7 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se)
 	dl_se->dl_defer			= 0;
 	dl_se->dl_defer_running		= 0;
 	dl_se->dl_defer_armed		= 0;
+	dl_se->dl_freq_invariant	= 0;

 #ifdef CONFIG_RT_MUTEXES
 	dl_se->pi_se			= dl_se;
--
2.56.0