linux-kernel - [PATCH 2/2] sched/cpupri: Remove pri_to

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Message-Id: <20200922083934.19275-3-dietmar.eggemann@arm.com>
Date:   Tue, 22 Sep 2020 10:39:34 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] sched/cpupri: Remove pri_to_cpu[1]

pri_to_cpu[1] isn't used since cpupri_set(..., newpri) is
never called with newpri = 99.

The valid RT priorities RT1..RT99 (p->rt_priority = [1..99]) map into
cpupri (idx of pri_to_cpu[]) = [2..100]

Current mapping:

p->rt_priority   p->prio   newpri   cpupri

                               -1       -1 (CPUPRI_INVALID)

                              100        0 (CPUPRI_NORMAL)

             1        98       98        2
           ...
            49        50       50       50
            50        49       49       51
           ...
            99         0        0      100

So cpupri = 1 isn't used.

Reduce the size of pri_to_cpu[] by 1 and adapt the cpupri
implementation accordingly. This will save a useless for loop with an
atomic_read in cpupri_find_fitness() calling __cpupri_find().

New mapping:

p->rt_priority   p->prio   newpri   cpupri

                               -1       -1 (CPUPRI_INVALID)

                              100        0 (CPUPRI_NORMAL)

             1        98       98        1
           ...
            49        50       50       49
            50        49       49       50
           ...
            99         0        0       99

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
 kernel/sched/cpupri.c | 6 +++---
 kernel/sched/cpupri.h | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index a5d14ed485f4..8d9952a51664 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -19,12 +19,12 @@
  *  in that class).  Therefore a typical application without affinity
  *  restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
  *  searches).  For tasks with affinity restrictions, the algorithm has a
- *  worst case complexity of O(min(101, nr_domcpus)), though the scenario that
+ *  worst case complexity of O(min(100, nr_domcpus)), though the scenario that
  *  yields the worst case search is fairly contrived.
  */
 #include "sched.h"
 
-/* Convert between a 140 based task->prio, and our 101 based cpupri */
+/* Convert between a 140 based task->prio, and our 100 based cpupri */
 static int convert_prio(int prio)
 {
 	int cpupri;
@@ -34,7 +34,7 @@ static int convert_prio(int prio)
 	else if (prio >= MAX_RT_PRIO)
 		cpupri = CPUPRI_NORMAL;
 	else
-		cpupri = MAX_RT_PRIO - prio;
+		cpupri = MAX_RT_PRIO - prio - 1;
 
 	return cpupri;
 }
diff --git a/kernel/sched/cpupri.h b/kernel/sched/cpupri.h
index 1a162369b8d4..e28e1ed12e3d 100644
--- a/kernel/sched/cpupri.h
+++ b/kernel/sched/cpupri.h
@@ -1,10 +1,10 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 
-#define CPUPRI_NR_PRIORITIES	(MAX_RT_PRIO + 1)
+#define CPUPRI_NR_PRIORITIES	MAX_RT_PRIO
 
 #define CPUPRI_INVALID		-1
 #define CPUPRI_NORMAL		 0
-/* values 2-100 are RT priorities 0-99 */
+/* values 1-99 are for RT1-RT99 priorities */
 
 struct cpupri_vec {
 	atomic_t		count;
-- 
2.17.1