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sched/cpupri: Remove pri_to_cpu[CPUPRI_IDLE]

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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 6e0534f278 ("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 57785df5ac ("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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200922083934.19275-2-dietmar.eggemann@arm.com
This commit is contained in:
Dietmar Eggemann 2020-09-22 10:39:33 +02:00 committed by Peter Zijlstra
parent a57415f5d1
commit 5e054bca44
2 changed files with 7 additions and 10 deletions
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10
kernel/sched/cpupri.c
View File

@ -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;
}

7
kernel/sched/cpupri.h
View File

@ -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;