Commit 1b08782c authored by Dietmar Eggemann's avatar Dietmar Eggemann Committed by Peter Zijlstra
Browse files

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: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200922083934.19275-3-dietmar.eggemann@arm.com
parent 5e054bca

kernel/sched/cpupri.c

+3 −3
Original line number Diff line number Diff line
@@ -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;

}

kernel/sched/cpupri.h

+2 −2
Original line number Diff line number Diff line 
/* 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;