linux-kernel - [PATCH 3/3] sched/deadline: Use sched_dl_entity's dl_density in dl_task_fits

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Message-Id: <20220629122053.287165-4-dietmar.eggemann@arm.com>
Date:   Wed, 29 Jun 2022 14:20:53 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Ben Segall <bsegall@...gle.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] sched/deadline: Use sched_dl_entity's dl_density in dl_task_fits_capacity()

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@....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