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) <[email protected]>
---
include/linux/sched.h | 5 ++++-
kernel/sched/core.c | 33 ++++++++++++++++++++++++++-------
kernel/sched/debug.c | 3 ++-
kernel/sched/fair.c | 6 ++++--
4 files changed, 36 insertions(+), 11 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -542,7 +542,10 @@ struct sched_entity {
struct list_head group_node;
unsigned int on_rq;
- unsigned int sched_delayed;
+
+ unsigned char sched_delayed;
+ unsigned char custom_slice;
+ /* 2 byte hole */
u64 exec_start;
u64 sum_exec_runtime;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4550,7 +4550,6 @@ 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);
#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -4801,6 +4800,8 @@ 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
@@ -7627,10 +7628,20 @@ 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
@@ -7812,7 +7823,9 @@ static int __sched_setscheduler(struct t
* 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 && attr->sched_runtime != p->se.slice)))
goto change;
if (rt_policy(policy) && attr->sched_priority != p->rt_priority)
goto change;
@@ -7958,6 +7971,9 @@ 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;
@@ -8124,12 +8140,14 @@ 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;
+ }
}
/**
@@ -10084,6 +10102,7 @@ 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
@@ -579,11 +579,12 @@ 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
@@ -984,7 +984,8 @@ 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
@@ -5169,7 +5170,8 @@ 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);
/*
On Fri, 05 Apr 2024 12:28:04 +0200 Peter Zijlstra <[email protected]>
> 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.
>
Given HZ=100 for example, what is preventing applications of suggested
slice=0.5ms from running 5ms a tick? If slice is 90ms otoh, is tick able
to kick the curr that has been on cpu for 10ms off cpu, given
cfs_rq->nr_running > 1?
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -984,7 +984,8 @@ 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