[PATCH 3/3] sched/deadline: Use sched_dl_entity's dl_density in dl_task_fits_capacity()

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

Save a multiplication in dl_task_fits_capacity() by using already
maintained per-sched_dl_entity (i.e. per-task) `dl_runtime/dl_deadline`
(dl_density).

  cap_scale(dl_deadline, cap) >= dl_runtime

  dl_deadline * cap >> SCHED_CAPACITY_SHIFT >= dl_runtime

  cap >= dl_runtime << SCHED_CAPACITY_SHIFT / dl_deadline

  with BW_SHIFT = 2 x SCHED_CAPACITY_SHIFT

  cap >= (dl_runtime << BW_SHIFT / dl_deadline) >> SCHED_CAPACITY_SHIFT

  cap >= dl_density >> SCHED_CAPACITY_SHIFT

__sched_setscheduler()->__checkparam_dl() ensures that the 2 corner
cases (if conditions) `runtime == RUNTIME_INF (-1)` and `period == 0`
of to_ratio(deadline, runtime) are not met when setting dl_density in
__sched_setscheduler()-> __setscheduler_params()->__setparam_dl().

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 kernel/sched/sched.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 79f65a6799c6..555f58d6c3a6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -324,15 +324,15 @@ struct dl_bw {
  * Verify the fitness of task @p to run on @cpu taking into account the
  * CPU original capacity and the runtime/deadline ratio of the task.
  *
- * The function will return true if the CPU original capacity of the
- * @cpu scaled by SCHED_CAPACITY_SCALE >= runtime/deadline ratio of the
- * task and false otherwise.
+ * The function will return true if the original capacity of @cpu is
+ * greater than or equal to task's deadline density right shifted by
+ * SCHED_CAPACITY_SHIFT and false otherwise.
  */
 static inline bool dl_task_fits_capacity(struct task_struct *p, int cpu)
 {
 	unsigned long cap = arch_scale_cpu_capacity(cpu);
 
-	return cap_scale(p->dl.dl_deadline, cap) >= p->dl.dl_runtime;
+	return cap >= p->dl.dl_density >> SCHED_CAPACITY_SHIFT;
 }
 
 extern void init_dl_bw(struct dl_bw *dl_b);
-- 
2.25.1