Commit 5e054bca authored by Dietmar Eggemann's avatar Dietmar Eggemann Committed by Peter Zijlstra
Browse files

sched/cpupri: Remove pri_to_cpu[CPUPRI_IDLE] 



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 6e0534f2 ("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 57785df5 ("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: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200922083934.19275-2-dietmar.eggemann@arm.com
parent a57415f5

kernel/sched/cpupri.c

+4 −6
Original line number Diff line number Diff line
@@ -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;

}

kernel/sched/cpupri.h

+3 −4
Original line number Diff line number Diff line 
/* 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;