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sched/eevdf: Use sched_attr::sched_runtime to set request/slice suggestion

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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@infradead.org>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105030.842834421@infradead.org
This commit is contained in:
Peter Zijlstra 2023-05-22 13:46:30 +02:00
parent 85e511df3c
commit 857b158dc5
5 changed files with 33 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/sched.h
View File

@ -547,6 +547,7 @@ struct sched_entity {
unsigned char on_rq;
unsigned char sched_delayed;
unsigned char rel_deadline;
unsigned char custom_slice;
/* hole */
u64 exec_start;

4
kernel/sched/core.c
View File

@ -4390,7 +4390,6 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
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(). */
@ -4643,6 +4642,8 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
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
@ -8412,6 +8413,7 @@ 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:

3
kernel/sched/debug.c
View File

@ -739,11 +739,12 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
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),

6
kernel/sched/fair.c
View File

@ -983,7 +983,8 @@ static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
* 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
@ -5227,7 +5228,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
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);
/*

29
kernel/sched/syscalls.c
View File

@ -401,10 +401,20 @@ static void __setscheduler_params(struct task_struct *p,
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 @@ recheck:
* 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 task_struct *p, int policy,
.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 @@ err_size:
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;
}
}
/**