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Subject[tip: sched/core] sched/cpupri: Remove pri_to_cpu[1]
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 1b08782ce31f612d98e11ccccf3e3df9a147a67d
Gitweb: https://git.kernel.org/tip/1b08782ce31f612d98e11ccccf3e3df9a147a67d
Author: Dietmar Eggemann <dietmar.eggemann@arm.com>
AuthorDate: Tue, 22 Sep 2020 10:39:34 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 29 Oct 2020 11:00:29 +01:00

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@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200922083934.19275-3-dietmar.eggemann@arm.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 a5d14ed..8d9952a 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 1a16236..e28e1ed 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;
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