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Subject[PATCH 1/2] sched/cpupri: Remove pri_to_cpu[CPUPRI_IDLE]
Date
pri_to_cpu[CPUPRI_IDLE=0] isn't used since cpupri_set(..., newpri) is
never called with newpri = MAX_PRIO (140).

Current mapping:

p->rt_priority p->prio newpri cpupri

-1 -1 (CPUPRI_INVALID)

140 0 (CPUPRI_IDLE)

100 1 (CPUPRI_NORMAL)

1 98 98 3
...
49 50 50 51
50 49 49 52
...
99 0 0 101

Even when cpupri was introduced with commit 6e0534f27819 ("sched: use a
2-d bitmap for searching lowest-pri CPU") in v2.6.27, only

(1) CPUPRI_INVALID (-1),
(2) MAX_RT_PRIO (100),
(3) an RT prio (RT1..RT99)

were used as newprio in cpupri_set(..., newpri) -> convert_prio(newpri).

MAX_RT_PRIO is used only in dec_rt_tasks() -> dec_rt_prio() ->
dec_rt_prio_smp() -> cpupri_set() in case of !rt_rq->rt_nr_running.
I.e. it stands for a non-rt task, including the IDLE task.

Commit 57785df5ac53 ("sched: Fix task priority bug") removed code in
v2.6.33 which did set the priority of the IDLE task to MAX_PRIO.
Although this happened after the introduction of cpupri, it didn't have
an effect on the values used for cpupri_set(..., newpri).

Remove CPUPRI_IDLE 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 2
...
49 50 50 50
50 49 49 51
...
99 0 0 100

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
kernel/sched/cpupri.c | 10 ++++------
kernel/sched/cpupri.h | 7 +++----
2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index 0033731a0797..a5d14ed485f4 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -11,7 +11,7 @@
* This code tracks the priority of each CPU so that global migration
* decisions are easy to calculate. Each CPU can be in a state as follows:
*
- * (INVALID), IDLE, NORMAL, RT1, ... RT99
+ * (INVALID), NORMAL, RT1, ... RT99
*
* going from the lowest priority to the highest. CPUs in the INVALID state
* are not eligible for routing. The system maintains this state with
@@ -19,24 +19,22 @@
* 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(102, nr_domcpus)), though the scenario that
+ * worst case complexity of O(min(101, 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 102 based cpupri */
+/* Convert between a 140 based task->prio, and our 101 based cpupri */
static int convert_prio(int prio)
{
int cpupri;

if (prio == CPUPRI_INVALID)
cpupri = CPUPRI_INVALID;
- else if (prio == MAX_PRIO)
- cpupri = CPUPRI_IDLE;
else if (prio >= MAX_RT_PRIO)
cpupri = CPUPRI_NORMAL;
else
- cpupri = MAX_RT_PRIO - prio + 1;
+ cpupri = MAX_RT_PRIO - prio;

return cpupri;
}
diff --git a/kernel/sched/cpupri.h b/kernel/sched/cpupri.h
index efbb492bb94c..1a162369b8d4 100644
--- a/kernel/sched/cpupri.h
+++ b/kernel/sched/cpupri.h
@@ -1,11 +1,10 @@
/* SPDX-License-Identifier: GPL-2.0 */

-#define CPUPRI_NR_PRIORITIES (MAX_RT_PRIO + 2)
+#define CPUPRI_NR_PRIORITIES (MAX_RT_PRIO + 1)

#define CPUPRI_INVALID -1
-#define CPUPRI_IDLE 0
-#define CPUPRI_NORMAL 1
-/* values 2-101 are RT priorities 0-99 */
+#define CPUPRI_NORMAL 0
+/* values 2-100 are RT priorities 0-99 */

struct cpupri_vec {
atomic_t count;
--
2.17.1
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