[PATCH 22/24] sched/eevdf: Use sched_attr::sched_runtime to set request/slice suggestion

From: Peter Zijlstra
Date: Sat Jul 27 2024 - 07:04:25 EST

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Allow applications to directly set a suggested request/slice length using
sched_attr::sched_runtime.

The implementation clamps the value to: 0.1[ms] <= slice <= 100[ms]
which is 1/10 the size of HZ=1000 and 10 times the size of HZ=100.

Applications should strive to use their periodic runtime at a high
confidence interval (95%+) as the target slice. Using a smaller slice
will introduce undue preemptions, while using a larger value will
increase latency.

For all the following examples assume a scheduling quantum of 8, and for
consistency all examples have W=4:

{A,B,C,D}(w=1,r=8):

ABCD...
+---+---+---+---

t=0, V=1.5 t=1, V=3.5
A |------< A |------<
B |------< B |------<
C |------< C |------<
D |------< D |------<
---+*------+-------+--- ---+--*----+-------+---

t=2, V=5.5 t=3, V=7.5
A |------< A |------<
B |------< B |------<
C |------< C |------<
D |------< D |------<
---+----*--+-------+--- ---+------*+-------+---

Note: 4 identical tasks in FIFO order

~~~

{A,B}(w=1,r=16) C(w=2,r=16)

AACCBBCC...
+---+---+---+---

t=0, V=1.25 t=2, V=5.25
A |--------------< A |--------------<
B |--------------< B |--------------<
C |------< C |------<
---+*------+-------+--- ---+----*--+-------+---

t=4, V=8.25 t=6, V=12.25
A |--------------< A |--------------<
B |--------------< B |--------------<
C |------< C |------<
---+-------*-------+--- ---+-------+---*---+---

Note: 1 heavy task -- because q=8, double r such that the deadline of the w=2
task doesn't go below q.

Note: observe the full schedule becomes: W*max(r_i/w_i) = 4*2q = 8q in length.

Note: the period of the heavy task is half the full period at:
W*(r_i/w_i) = 4*(2q/2) = 4q

~~~

{A,C,D}(w=1,r=16) B(w=1,r=8):

BAACCBDD...
+---+---+---+---

t=0, V=1.5 t=1, V=3.5
A |--------------< A |---------------<
B |------< B |------<
C |--------------< C |--------------<
D |--------------< D |--------------<
---+*------+-------+--- ---+--*----+-------+---

t=3, V=7.5 t=5, V=11.5
A |---------------< A |---------------<
B |------< B |------<
C |--------------< C |--------------<
D |--------------< D |--------------<
---+------*+-------+--- ---+-------+--*----+---

t=6, V=13.5
A |---------------<
B |------<
C |--------------<
D |--------------<
---+-------+----*--+---

Note: 1 short task -- again double r so that the deadline of the short task
won't be below q. Made B short because its not the leftmost task, but is
eligible with the 0,1,2,3 spread.

Note: like with the heavy task, the period of the short task observes:
W*(r_i/w_i) = 4*(1q/1) = 4q

~~~

A(w=1,r=16) B(w=1,r=8) C(w=2,r=16)

BCCAABCC...
+---+---+---+---

t=0, V=1.25 t=1, V=3.25
A |--------------< A |--------------<
B |------< B |------<
C |------< C |------<
---+*------+-------+--- ---+--*----+-------+---

t=3, V=7.25 t=5, V=11.25
A |--------------< A |--------------<
B |------< B |------<
C |------< C |------<
---+------*+-------+--- ---+-------+--*----+---

t=6, V=13.25
A |--------------<
B |------<
C |------<
---+-------+----*--+---

Note: 1 heavy and 1 short task -- combine them all.

Note: both the short and heavy task end up with a period of 4q

~~~

A(w=1,r=16) B(w=2,r=16) C(w=1,r=8)

BBCAABBC...
+---+---+---+---

t=0, V=1 t=2, V=5
A |--------------< A |--------------<
B |------< B |------<
C |------< C |------<
---+*------+-------+--- ---+----*--+-------+---

t=3, V=7 t=5, V=11
A |--------------< A |--------------<
B |------< B |------<
C |------< C |------<
---+------*+-------+--- ---+-------+--*----+---

t=7, V=15
A |--------------<
B |------<
C |------<
---+-------+------*+---

Note: as before but permuted

~~~

>From all this it can be deduced that, for the steady state:

- the total period (P) of a schedule is: W*max(r_i/w_i)
- the average period of a task is: W*(r_i/w_i)
- each task obtains the fair share: w_i/W of each full period P

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 4 +++-
kernel/sched/debug.c | 3 ++-
kernel/sched/fair.c | 6 ++++--
kernel/sched/syscalls.c | 29 +++++++++++++++++++++++------
5 files changed, 33 insertions(+), 10 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -544,6 +544,7 @@ struct sched_entity {
unsigned char on_rq;
unsigned char sched_delayed;
unsigned char rel_deadline;
+ unsigned char custom_slice;
/* hole */

u64 exec_start;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4347,7 +4347,6 @@ static void __sched_fork(unsigned long c
p->se.nr_migrations = 0;
p->se.vruntime = 0;
p->se.vlag = 0;
- p->se.slice = sysctl_sched_base_slice;
INIT_LIST_HEAD(&p->se.group_node);

/* A delayed task cannot be in clone(). */
@@ -4600,6 +4599,8 @@ int sched_fork(unsigned long clone_flags

p->prio = p->normal_prio = p->static_prio;
set_load_weight(p, false);
+ p->se.custom_slice = 0;
+ p->se.slice = sysctl_sched_base_slice;

/*
* We don't need the reset flag anymore after the fork. It has
@@ -8328,6 +8329,7 @@ void __init sched_init(void)
}

set_load_weight(&init_task, false);
+ init_task.se.slice = sysctl_sched_base_slice,

/*
* The boot idle thread does lazy MMU switching as well:
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -580,11 +580,12 @@ print_task(struct seq_file *m, struct rq
else
SEQ_printf(m, " %c", task_state_to_char(p));

- SEQ_printf(m, "%15s %5d %9Ld.%06ld %c %9Ld.%06ld %9Ld.%06ld %9Ld.%06ld %9Ld %5d ",
+ SEQ_printf(m, "%15s %5d %9Ld.%06ld %c %9Ld.%06ld %c %9Ld.%06ld %9Ld.%06ld %9Ld %5d ",
p->comm, task_pid_nr(p),
SPLIT_NS(p->se.vruntime),
entity_eligible(cfs_rq_of(&p->se), &p->se) ? 'E' : 'N',
SPLIT_NS(p->se.deadline),
+ p->se.custom_slice ? 'S' : ' ',
SPLIT_NS(p->se.slice),
SPLIT_NS(p->se.sum_exec_runtime),
(long long)(p->nvcsw + p->nivcsw),
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -995,7 +995,8 @@ static void update_deadline(struct cfs_r
* nice) while the request time r_i is determined by
* sysctl_sched_base_slice.
*/
- se->slice = sysctl_sched_base_slice;
+ if (!se->custom_slice)
+ se->slice = sysctl_sched_base_slice;

/*
* EEVDF: vd_i = ve_i + r_i / w_i
@@ -5190,7 +5191,8 @@ place_entity(struct cfs_rq *cfs_rq, stru
u64 vslice, vruntime = avg_vruntime(cfs_rq);
s64 lag = 0;

- se->slice = sysctl_sched_base_slice;
+ if (!se->custom_slice)
+ se->slice = sysctl_sched_base_slice;
vslice = calc_delta_fair(se->slice, se);

/*
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -401,10 +401,20 @@ static void __setscheduler_params(struct

p->policy = policy;

- if (dl_policy(policy))
+ if (dl_policy(policy)) {
__setparam_dl(p, attr);
- else if (fair_policy(policy))
+ } else if (fair_policy(policy)) {
p->static_prio = NICE_TO_PRIO(attr->sched_nice);
+ if (attr->sched_runtime) {
+ p->se.custom_slice = 1;
+ p->se.slice = clamp_t(u64, attr->sched_runtime,
+ NSEC_PER_MSEC/10, /* HZ=1000 * 10 */
+ NSEC_PER_MSEC*100); /* HZ=100 / 10 */
+ } else {
+ p->se.custom_slice = 0;
+ p->se.slice = sysctl_sched_base_slice;
+ }
+ }

/*
* __sched_setscheduler() ensures attr->sched_priority == 0 when
@@ -700,7 +710,9 @@ int __sched_setscheduler(struct task_str
* but store a possible modification of reset_on_fork.
*/
if (unlikely(policy == p->policy)) {
- if (fair_policy(policy) && attr->sched_nice != task_nice(p))
+ if (fair_policy(policy) &&
+ (attr->sched_nice != task_nice(p) ||
+ (attr->sched_runtime != p->se.slice)))
goto change;
if (rt_policy(policy) && attr->sched_priority != p->rt_priority)
goto change;
@@ -846,6 +858,9 @@ static int _sched_setscheduler(struct ta
.sched_nice = PRIO_TO_NICE(p->static_prio),
};

+ if (p->se.custom_slice)
+ attr.sched_runtime = p->se.slice;
+
/* Fixup the legacy SCHED_RESET_ON_FORK hack. */
if ((policy != SETPARAM_POLICY) && (policy & SCHED_RESET_ON_FORK)) {
attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
@@ -1012,12 +1027,14 @@ static int sched_copy_attr(struct sched_

static void get_params(struct task_struct *p, struct sched_attr *attr)
{
- if (task_has_dl_policy(p))
+ if (task_has_dl_policy(p)) {
__getparam_dl(p, attr);
- else if (task_has_rt_policy(p))
+ } else if (task_has_rt_policy(p)) {
attr->sched_priority = p->rt_priority;
- else
+ } else {
attr->sched_nice = task_nice(p);
+ attr->sched_runtime = p->se.slice;
+ }
}

/**

Next message: Peter Zijlstra: "[RFC PATCH 24/24] sched/time: Introduce CLOCK_THREAD_DVFS_ID"
Previous message: Peter Zijlstra: "[PATCH 16/24] sched: Teach dequeue_task() about special task states"
In reply to: Peter Zijlstra: "[PATCH 16/24] sched: Teach dequeue_task() about special task states"
Next in thread: Peter Zijlstra: "[RFC PATCH 24/24] sched/time: Introduce CLOCK_THREAD_DVFS_ID"
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