[PATCH 0/3] sched: Task priority related cleanups

[PATCH 0/3] sched: Task priority related cleanups

[Dietmar Eggemann]

(1) Removing MAX_USER_RT_PRIO was already discussed here in April 2020:

    https://lkml.kernel.org/r/20200423094403.6f1d2b8d@gandalf.local.home

(2) USER_PRIO() and related macros are not used anymore except in one
    case for powerpc where MAX_USER_PRIO can be replaced by NICE_WIDTH.
    Set_load_weight(), task_prio(), cpu_weight_nice_write_s64(),
    __update_max_tr() don't use USER_PRIO() but priority - MAX_RT_PRIO.

(3) The function header of task_prio() needs an update. It looks
    ancient since it mentions a prio space [-16 ... 15] for mormal
    tasks. I can't figure out why this range is mentioned here? Maybe
    the influence of the 'sleep-bonus interactivity' feature which was
    removed by commit f3479f10c5d6 ("sched: remove the sleep-bonus
    interactivity code")? 

Dietmar Eggemann (3):
  sched: Remove MAX_USER_RT_PRIO
  sched: Remove USER_PRIO, TASK_USER_PRIO and MAX_USER_PRIO
  sched/core: Update task_prio() function header

 arch/powerpc/platforms/cell/spufs/sched.c |  2 +-
 include/linux/sched/prio.h                | 18 +-----------------
 kernel/sched/core.c                       | 15 +++++++++------
 kernel/sched/sched.h                      |  2 +-
 4 files changed, 12 insertions(+), 25 deletions(-)

-- 
2.25.1

[PATCH 1/3] sched: Remove MAX_USER_RT_PRIO

[Dietmar Eggemann]

Commit d46523ea32a7 ("[PATCH] fix MAX_USER_RT_PRIO and MAX_RT_PRIO")
was introduced due to a a small time period in which the realtime patch
set was using different values for MAX_USER_RT_PRIO and MAX_RT_PRIO.

This is no longer true, i.e. now MAX_RT_PRIO == MAX_USER_RT_PRIO.

Get rid of MAX_USER_RT_PRIO and make everything use MAX_RT_PRIO
instead.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 include/linux/sched/prio.h | 9 +--------
 kernel/sched/core.c        | 7 +++----
 2 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
index 7d64feafc408..d111f2fd77ea 100644
--- a/include/linux/sched/prio.h
+++ b/include/linux/sched/prio.h
@@ -11,16 +11,9 @@
  * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
  * tasks are in the range MAX_RT_PRIO..MAX_PRIO-1. Priority
  * values are inverted: lower p->prio value means higher priority.
- *
- * The MAX_USER_RT_PRIO value allows the actual maximum
- * RT priority to be separate from the value exported to
- * user-space.  This allows kernel threads to set their
- * priority to a value higher than any user task. Note:
- * MAX_RT_PRIO must not be smaller than MAX_USER_RT_PRIO.
  */
 
-#define MAX_USER_RT_PRIO	100
-#define MAX_RT_PRIO		MAX_USER_RT_PRIO
+#define MAX_RT_PRIO		100
 
 #define MAX_PRIO		(MAX_RT_PRIO + NICE_WIDTH)
 #define DEFAULT_PRIO		(MAX_RT_PRIO + NICE_WIDTH / 2)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 06b449942adf..625ec1e12064 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5897,11 +5897,10 @@ static int __sched_setscheduler(struct task_struct *p,
 
 	/*
 	 * Valid priorities for SCHED_FIFO and SCHED_RR are
-	 * 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL,
+	 * 1..MAX_RT_PRIO-1, valid priority for SCHED_NORMAL,
 	 * SCHED_BATCH and SCHED_IDLE is 0.
 	 */
-	if ((p->mm && attr->sched_priority > MAX_USER_RT_PRIO-1) ||
-	    (!p->mm && attr->sched_priority > MAX_RT_PRIO-1))
+	if (attr->sched_priority > MAX_RT_PRIO-1)
 		return -EINVAL;
 	if ((dl_policy(policy) && !__checkparam_dl(attr)) ||
 	    (rt_policy(policy) != (attr->sched_priority != 0)))
@@ -6969,7 +6968,7 @@ SYSCALL_DEFINE1(sched_get_priority_max, int, policy)
 	switch (policy) {
 	case SCHED_FIFO:
 	case SCHED_RR:
-		ret = MAX_USER_RT_PRIO-1;
+		ret = MAX_RT_PRIO-1;
 		break;
 	case SCHED_DEADLINE:
 	case SCHED_NORMAL:
-- 
2.25.1

[PATCH 2/3] sched: Remove USER_PRIO, TASK_USER_PRIO and MAX_USER_PRIO

[Dietmar Eggemann]

The only remaining use of MAX_USER_PRIO (and USER_PRIO) is the
SCALE_PRIO() definition in the PowerPC Cell architecture's Synergistic
Processor Unit (SPU) scheduler. TASK_USER_PRIO isn't used anymore.

Commit fe443ef2ac42 ("[POWERPC] spusched: Dynamic timeslicing for
SCHED_OTHER") copied SCALE_PRIO() from the task scheduler in v2.6.23.

Commit a4ec24b48dde ("sched: tidy up SCHED_RR") removed it from the task
scheduler in v2.6.24.

Commit 3ee237dddcd8 ("sched/prio: Add 3 macros of MAX_NICE, MIN_NICE and
NICE_WIDTH in prio.h") introduced NICE_WIDTH much later.

With:

  MAX_USER_PRIO = USER_PRIO(MAX_PRIO)

                = MAX_PRIO - MAX_RT_PRIO

       MAX_PRIO = MAX_RT_PRIO + NICE_WIDTH

  MAX_USER_PRIO = MAX_RT_PRIO + NICE_WIDTH - MAX_RT_PRIO

  MAX_USER_PRIO = NICE_WIDTH

MAX_USER_PRIO can be replaced by NICE_WIDTH to be able to remove all the
{*_}USER_PRIO defines.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 arch/powerpc/platforms/cell/spufs/sched.c | 2 +-
 include/linux/sched/prio.h                | 9 ---------
 kernel/sched/sched.h                      | 2 +-
 3 files changed, 2 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index f18d5067cd0f..aeb7f3922106 100644
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/arch/powerpc/platforms/cell/spufs/sched.c
@@ -72,7 +72,7 @@ static struct timer_list spuloadavg_timer;
 #define DEF_SPU_TIMESLICE	(100 * HZ / (1000 * SPUSCHED_TICK))
 
 #define SCALE_PRIO(x, prio) \
-	max(x * (MAX_PRIO - prio) / (MAX_USER_PRIO / 2), MIN_SPU_TIMESLICE)
+	max(x * (MAX_PRIO - prio) / (NICE_WIDTH / 2), MIN_SPU_TIMESLICE)
 
 /*
  * scale user-nice values [ -20 ... 0 ... 19 ] to time slice values:
diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
index d111f2fd77ea..ab83d85e1183 100644
--- a/include/linux/sched/prio.h
+++ b/include/linux/sched/prio.h
@@ -26,15 +26,6 @@
 #define NICE_TO_PRIO(nice)	((nice) + DEFAULT_PRIO)
 #define PRIO_TO_NICE(prio)	((prio) - DEFAULT_PRIO)
 
-/*
- * 'User priority' is the nice value converted to something we
- * can work with better when scaling various scheduler parameters,
- * it's a [ 0 ... 39 ] range.
- */
-#define USER_PRIO(p)		((p)-MAX_RT_PRIO)
-#define TASK_USER_PRIO(p)	USER_PRIO((p)->static_prio)
-#define MAX_USER_PRIO		(USER_PRIO(MAX_PRIO))
-
 /*
  * Convert nice value [19,-20] to rlimit style value [1,40].
  */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 045b01064c1e..6edc67df3554 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -140,7 +140,7 @@ extern void call_trace_sched_update_nr_running(struct rq *rq, int count);
  * scale_load() and scale_load_down(w) to convert between them. The
  * following must be true:
  *
- *  scale_load(sched_prio_to_weight[USER_PRIO(NICE_TO_PRIO(0))]) == NICE_0_LOAD
+ *  scale_load(sched_prio_to_weight[NICE_TO_PRIO(0)-MAX_RT_PRIO]) == NICE_0_LOAD
  *
  */
 #define NICE_0_LOAD		(1L << NICE_0_LOAD_SHIFT)
-- 
2.25.1

[PATCH 3/3] sched/core: Update task_prio() function header

[Dietmar Eggemann]

The description of the RT offset and the values for 'normal' tasks needs
update. Moreover there are DL tasks now.
task_prio() has to stay like it is to guarantee compatibility with the
/proc/<pid>/stat priority field:

  # cat /proc/<pid>/stat | awk '{ print $18; }'

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 kernel/sched/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 625ec1e12064..be3a956c2d23 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5602,8 +5602,12 @@ SYSCALL_DEFINE1(nice, int, increment)
  * @p: the task in question.
  *
  * Return: The priority value as seen by users in /proc.
- * RT tasks are offset by -200. Normal tasks are centered
- * around 0, value goes from -16 to +15.
+ *
+ * sched policy         return value   kernel prio    user prio/nice
+ *
+ * normal, batch, idle     [0 ... 39]  [100 ... 139]          0/[-20 ... 19]
+ * fifo, rr             [-2 ... -100]     [98 ... 0]  [1 ... 99]
+ * deadline                     -101             -1           0
  */
 int task_prio(const struct task_struct *p)
 {
-- 
2.25.1

Re: [PATCH 0/3] sched: Task priority related cleanups

[Peter Zijlstra]

On Thu, Jan 28, 2021 at 02:10:37PM +0100, Dietmar Eggemann wrote:

> Dietmar Eggemann (3):
>   sched: Remove MAX_USER_RT_PRIO
>   sched: Remove USER_PRIO, TASK_USER_PRIO and MAX_USER_PRIO
>   sched/core: Update task_prio() function header

Thanks!