Messages in this thread Patch in this message | | | Date | Fri, 05 Apr 2024 12:28:04 +0200 | From | Peter Zijlstra <> | Subject | [RFC][PATCH 10/10] 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> --- 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); /*
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