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Message-ID: <CAKfTPtDDBVD_N6NgBYi_5iArDXd4iL0-ddQZDKGzzLAD-2AUXg@mail.gmail.com>
Date: Wed, 29 Mar 2023 16:35:25 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, linux-kernel@...r.kernel.org,
juri.lelli@...hat.com, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, corbet@....net, qyousef@...alina.io,
chris.hyser@...cle.com, patrick.bellasi@...bug.net, pjt@...gle.com,
pavel@....cz, qperret@...gle.com, tim.c.chen@...ux.intel.com,
joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
yu.c.chen@...el.com, youssefesmat@...omium.org,
joel@...lfernandes.org, efault@....de
Subject: Re: [PATCH 08/17] sched/fair: Implement an EEVDF like policy
On Tue, 28 Mar 2023 at 13:06, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Where CFS is currently a WFQ based scheduler with only a single knob,
> the weight. The addition of a second, latency oriented parameter,
> makes something like WF2Q or EEVDF based a much better fit.
>
> Specifically, EEVDF does EDF like scheduling in the left half of the
> tree -- those entities that are owed service. Except because this is a
> virtual time scheduler, the deadlines are in virtual time as well,
> which is what allows over-subscription.
>
> EEVDF has two parameters:
>
> - weight, or time-slope; which is mapped to nice just as before
> - relative deadline; which is related to slice length and mapped
> to the new latency nice.
>
> Basically, by setting a smaller slice, the deadline will be earlier
> and the task will be more eligible and ran earlier.
IIUC how it works, Vd = ve + r / wi
So for a same weight, the vd will be earlier but it's no more alway
true for different weight
>
> Preemption (both tick and wakeup) is driven by testing against a fresh
> pick. Because the tree is now effectively an interval tree, and the
> selection is no longer 'leftmost', over-scheduling is less of a
> problem.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> include/linux/sched.h | 4
> kernel/sched/debug.c | 6
> kernel/sched/fair.c | 324 +++++++++++++++++++++++++++++++++++++++++-------
> kernel/sched/features.h | 3
> kernel/sched/sched.h | 1
> 5 files changed, 293 insertions(+), 45 deletions(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -548,6 +548,9 @@ struct sched_entity {
> /* For load-balancing: */
> struct load_weight load;
> struct rb_node run_node;
> + u64 deadline;
> + u64 min_deadline;
> +
> struct list_head group_node;
> unsigned int on_rq;
>
> @@ -556,6 +559,7 @@ struct sched_entity {
> u64 vruntime;
> u64 prev_sum_exec_runtime;
> s64 vlag;
> + u64 slice;
>
> u64 nr_migrations;
>
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -535,9 +535,13 @@ 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 %9Ld %5d ",
> + SEQ_printf(m, "%15s %5d %9Ld.%06ld %c %9Ld.%06ld %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),
> + SPLIT_NS(p->se.slice),
> + SPLIT_NS(p->se.sum_exec_runtime),
> (long long)(p->nvcsw + p->nivcsw),
> p->prio);
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -47,6 +47,7 @@
> #include <linux/psi.h>
> #include <linux/ratelimit.h>
> #include <linux/task_work.h>
> +#include <linux/rbtree_augmented.h>
>
> #include <asm/switch_to.h>
>
> @@ -347,6 +348,16 @@ static u64 __calc_delta(u64 delta_exec,
> return mul_u64_u32_shr(delta_exec, fact, shift);
> }
>
> +/*
> + * delta /= w
> + */
> +static inline u64 calc_delta_fair(u64 delta, struct sched_entity *se)
> +{
> + if (unlikely(se->load.weight != NICE_0_LOAD))
> + delta = __calc_delta(delta, NICE_0_LOAD, &se->load);
> +
> + return delta;
> +}
>
> const struct sched_class fair_sched_class;
>
> @@ -691,11 +702,62 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
>
> /*
> * lag_i = S - s_i = w_i * (V - v_i)
> + *
> + * However, since V is approximated by the weighted average of all entities it
> + * is possible -- by addition/removal/reweight to the tree -- to move V around
> + * and end up with a larger lag than we started with.
> + *
> + * Limit this to either double the slice length with a minimum of TICK_NSEC
> + * since that is the timing granularity.
> + *
> + * EEVDF gives the following limit for a steady state system:
> + *
> + * -r_max < lag < max(r_max, q)
> + *
> + * XXX could add max_slice to the augmented data to track this.
> */
> void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> + s64 lag, limit;
> +
> SCHED_WARN_ON(!se->on_rq);
> - se->vlag = avg_vruntime(cfs_rq) - se->vruntime;
> + lag = avg_vruntime(cfs_rq) - se->vruntime;
> +
> + limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
> + se->vlag = clamp(lag, -limit, limit);
> +}
> +
> +/*
> + * Entity is eligible once it received less service than it ought to have,
> + * eg. lag >= 0.
> + *
> + * lag_i = S - s_i = w_i*(V - v_i)
> + *
> + * lag_i >= 0 -> V >= v_i
> + *
> + * \Sum (v_i - v)*w_i
> + * V = ------------------ + v
> + * \Sum w_i
> + *
> + * lag_i >= 0 -> \Sum (v_i - v)*w_i >= (v_i - v)*(\Sum w_i)
> + *
> + * Note: using 'avg_vruntime() > se->vruntime' is inacurate due
> + * to the loss in precision caused by the division.
> + */
> +int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> + struct sched_entity *curr = cfs_rq->curr;
> + s64 avg = cfs_rq->avg_vruntime;
> + long load = cfs_rq->avg_load;
> +
> + if (curr && curr->on_rq) {
> + unsigned long weight = scale_load_down(curr->load.weight);
> +
> + avg += entity_key(cfs_rq, curr) * weight;
> + load += weight;
> + }
> +
> + return avg >= entity_key(cfs_rq, se) * load;
> }
>
> static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
> @@ -714,8 +776,8 @@ static u64 __update_min_vruntime(struct
>
> static void update_min_vruntime(struct cfs_rq *cfs_rq)
> {
> + struct sched_entity *se = __pick_first_entity(cfs_rq);
> struct sched_entity *curr = cfs_rq->curr;
> - struct rb_node *leftmost = rb_first_cached(&cfs_rq->tasks_timeline);
>
> u64 vruntime = cfs_rq->min_vruntime;
>
> @@ -726,9 +788,7 @@ static void update_min_vruntime(struct c
> curr = NULL;
> }
>
> - if (leftmost) { /* non-empty tree */
> - struct sched_entity *se = __node_2_se(leftmost);
> -
> + if (se) {
> if (!curr)
> vruntime = se->vruntime;
> else
> @@ -745,18 +805,50 @@ static inline bool __entity_less(struct
> return entity_before(__node_2_se(a), __node_2_se(b));
> }
>
> +#define deadline_gt(field, lse, rse) ({ (s64)((lse)->field - (rse)->field) > 0; })
> +
> +static inline void __update_min_deadline(struct sched_entity *se, struct rb_node *node)
> +{
> + if (node) {
> + struct sched_entity *rse = __node_2_se(node);
> + if (deadline_gt(min_deadline, se, rse))
> + se->min_deadline = rse->min_deadline;
> + }
> +}
> +
> +/*
> + * se->min_deadline = min(se->deadline, left->min_deadline, right->min_deadline)
> + */
> +static inline bool min_deadline_update(struct sched_entity *se, bool exit)
> +{
> + u64 old_min_deadline = se->min_deadline;
> + struct rb_node *node = &se->run_node;
> +
> + se->min_deadline = se->deadline;
> + __update_min_deadline(se, node->rb_right);
> + __update_min_deadline(se, node->rb_left);
> +
> + return se->min_deadline == old_min_deadline;
> +}
> +
> +RB_DECLARE_CALLBACKS(static, min_deadline_cb, struct sched_entity,
> + run_node, min_deadline, min_deadline_update);
> +
> /*
> * Enqueue an entity into the rb-tree:
> */
> static void __enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> avg_vruntime_add(cfs_rq, se);
> - rb_add_cached(&se->run_node, &cfs_rq->tasks_timeline, __entity_less);
> + se->min_deadline = se->deadline;
> + rb_add_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
> + __entity_less, &min_deadline_cb);
> }
>
> static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> - rb_erase_cached(&se->run_node, &cfs_rq->tasks_timeline);
> + rb_erase_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
> + &min_deadline_cb);
> avg_vruntime_sub(cfs_rq, se);
> }
>
> @@ -780,6 +872,97 @@ static struct sched_entity *__pick_next_
> return __node_2_se(next);
> }
>
> +static struct sched_entity *pick_cfs(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> +{
> + struct sched_entity *left = __pick_first_entity(cfs_rq);
> +
> + /*
> + * If curr is set we have to see if its left of the leftmost entity
> + * still in the tree, provided there was anything in the tree at all.
> + */
> + if (!left || (curr && entity_before(curr, left)))
> + left = curr;
> +
> + return left;
> +}
> +
> +/*
> + * Earliest Eligible Virtual Deadline First
> + *
> + * In order to provide latency guarantees for different request sizes
> + * EEVDF selects the best runnable task from two criteria:
> + *
> + * 1) the task must be eligible (must be owed service)
> + *
> + * 2) from those tasks that meet 1), we select the one
> + * with the earliest virtual deadline.
> + *
> + * We can do this in O(log n) time due to an augmented RB-tree. The
> + * tree keeps the entries sorted on service, but also functions as a
> + * heap based on the deadline by keeping:
> + *
> + * se->min_deadline = min(se->deadline, se->{left,right}->min_deadline)
> + *
> + * Which allows an EDF like search on (sub)trees.
> + */
> +static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
> +{
> + struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
> + struct sched_entity *curr = cfs_rq->curr;
> + struct sched_entity *best = NULL;
> +
> + if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
> + curr = NULL;
> +
> + while (node) {
> + struct sched_entity *se = __node_2_se(node);
> +
> + /*
> + * If this entity is not eligible, try the left subtree.
> + */
> + if (!entity_eligible(cfs_rq, se)) {
> + node = node->rb_left;
> + continue;
> + }
> +
> + /*
> + * If this entity has an earlier deadline than the previous
> + * best, take this one. If it also has the earliest deadline
> + * of its subtree, we're done.
> + */
> + if (!best || deadline_gt(deadline, best, se)) {
> + best = se;
> + if (best->deadline == best->min_deadline)
> + break;
> + }
> +
> + /*
> + * If the earlest deadline in this subtree is in the fully
> + * eligible left half of our space, go there.
> + */
> + if (node->rb_left &&
> + __node_2_se(node->rb_left)->min_deadline == se->min_deadline) {
> + node = node->rb_left;
> + continue;
> + }
> +
> + node = node->rb_right;
> + }
> +
> + if (!best || (curr && deadline_gt(deadline, best, curr)))
> + best = curr;
> +
> + if (unlikely(!best)) {
> + struct sched_entity *left = __pick_first_entity(cfs_rq);
> + if (left) {
> + pr_err("EEVDF scheduling fail, picking leftmost\n");
> + return left;
> + }
> + }
> +
> + return best;
> +}
> +
> #ifdef CONFIG_SCHED_DEBUG
> struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
> {
> @@ -822,17 +1005,6 @@ long calc_latency_offset(int prio)
> }
>
> /*
> - * delta /= w
> - */
> -static inline u64 calc_delta_fair(u64 delta, struct sched_entity *se)
> -{
> - if (unlikely(se->load.weight != NICE_0_LOAD))
> - delta = __calc_delta(delta, NICE_0_LOAD, &se->load);
> -
> - return delta;
> -}
> -
> -/*
> * The idea is to set a period in which each task runs once.
> *
> * When there are too many tasks (sched_nr_latency) we have to stretch
> @@ -897,6 +1069,38 @@ static u64 sched_slice(struct cfs_rq *cf
> return slice;
> }
>
> +/*
> + * XXX: strictly: vd_i += N*r_i/w_i such that: vd_i > ve_i
> + * this is probably good enough.
> + */
> +static void update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> + if ((s64)(se->vruntime - se->deadline) < 0)
> + return;
> +
> + if (sched_feat(EEVDF)) {
> + /*
> + * For EEVDF the virtual time slope is determined by w_i (iow.
> + * nice) while the request time r_i is determined by
> + * latency-nice.
> + */
> + se->slice = se->latency_offset;
> + } else {
> + /*
> + * When many tasks blow up the sched_period; it is possible
> + * that sched_slice() reports unusually large results (when
> + * many tasks are very light for example). Therefore impose a
> + * maximum.
> + */
> + se->slice = min_t(u64, sched_slice(cfs_rq, se), sysctl_sched_latency);
> + }
> +
> + /*
> + * EEVDF: vd_i = ve_i + r_i / w_i
> + */
> + se->deadline = se->vruntime + calc_delta_fair(se->slice, se);
> +}
> +
> #include "pelt.h"
> #ifdef CONFIG_SMP
>
> @@ -1029,6 +1233,7 @@ static void update_curr(struct cfs_rq *c
> schedstat_add(cfs_rq->exec_clock, delta_exec);
>
> curr->vruntime += calc_delta_fair(delta_exec, curr);
> + update_deadline(cfs_rq, curr);
> update_min_vruntime(cfs_rq);
>
> if (entity_is_task(curr)) {
> @@ -4796,6 +5001,7 @@ static inline bool entity_is_long_sleepe
> static void
> place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> {
> + u64 vslice = calc_delta_fair(se->slice, se);
> u64 vruntime = avg_vruntime(cfs_rq);
> s64 lag = 0;
>
> @@ -4834,9 +5040,9 @@ place_entity(struct cfs_rq *cfs_rq, stru
> */
> load = cfs_rq->avg_load;
> if (curr && curr->on_rq)
> - load += curr->load.weight;
> + load += scale_load_down(curr->load.weight);
>
> - lag *= load + se->load.weight;
> + lag *= load + scale_load_down(se->load.weight);
> if (WARN_ON_ONCE(!load))
> load = 1;
> lag = div_s64(lag, load);
> @@ -4877,6 +5083,19 @@ place_entity(struct cfs_rq *cfs_rq, stru
> }
>
> se->vruntime = vruntime;
> +
> + /*
> + * When joining the competition; the exisiting tasks will be,
> + * on average, halfway through their slice, as such start tasks
> + * off with half a slice to ease into the competition.
> + */
> + if (sched_feat(PLACE_DEADLINE_INITIAL) && initial)
> + vslice /= 2;
> +
> + /*
> + * EEVDF: vd_i = ve_i + r_i/w_i
> + */
> + se->deadline = se->vruntime + vslice;
> }
>
> static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
> @@ -5088,19 +5307,20 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> static void
> check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> {
> - unsigned long ideal_runtime, delta_exec;
> + unsigned long delta_exec;
> struct sched_entity *se;
> s64 delta;
>
> - /*
> - * When many tasks blow up the sched_period; it is possible that
> - * sched_slice() reports unusually large results (when many tasks are
> - * very light for example). Therefore impose a maximum.
> - */
> - ideal_runtime = min_t(u64, sched_slice(cfs_rq, curr), sysctl_sched_latency);
> + if (sched_feat(EEVDF)) {
> + if (pick_eevdf(cfs_rq) != curr)
> + goto preempt;
> +
> + return;
> + }
>
> delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> - if (delta_exec > ideal_runtime) {
> + if (delta_exec > curr->slice) {
> +preempt:
> resched_curr(rq_of(cfs_rq));
> /*
> * The current task ran long enough, ensure it doesn't get
> @@ -5124,7 +5344,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq
> if (delta < 0)
> return;
>
> - if (delta > ideal_runtime)
> + if (delta > curr->slice)
> resched_curr(rq_of(cfs_rq));
> }
>
> @@ -5179,17 +5399,20 @@ wakeup_preempt_entity(struct sched_entit
> static struct sched_entity *
> pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> {
> - struct sched_entity *left = __pick_first_entity(cfs_rq);
> - struct sched_entity *se;
> + struct sched_entity *left, *se;
>
> - /*
> - * If curr is set we have to see if its left of the leftmost entity
> - * still in the tree, provided there was anything in the tree at all.
> - */
> - if (!left || (curr && entity_before(curr, left)))
> - left = curr;
> + if (sched_feat(EEVDF)) {
> + /*
> + * Enabling NEXT_BUDDY will affect latency but not fairness.
> + */
> + if (sched_feat(NEXT_BUDDY) &&
> + cfs_rq->next && entity_eligible(cfs_rq, cfs_rq->next))
> + return cfs_rq->next;
> +
> + return pick_eevdf(cfs_rq);
> + }
>
> - se = left; /* ideally we run the leftmost entity */
> + se = left = pick_cfs(cfs_rq, curr);
>
> /*
> * Avoid running the skip buddy, if running something else can
> @@ -6284,13 +6507,12 @@ static inline void unthrottle_offline_cf
> static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
> {
> struct sched_entity *se = &p->se;
> - struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> SCHED_WARN_ON(task_rq(p) != rq);
>
> if (rq->cfs.h_nr_running > 1) {
> - u64 slice = sched_slice(cfs_rq, se);
> u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> + u64 slice = se->slice;
> s64 delta = slice - ran;
>
> if (delta < 0) {
> @@ -8010,7 +8232,19 @@ static void check_preempt_wakeup(struct
> if (cse_is_idle != pse_is_idle)
> return;
>
> - update_curr(cfs_rq_of(se));
> + cfs_rq = cfs_rq_of(se);
> + update_curr(cfs_rq);
> +
> + if (sched_feat(EEVDF)) {
> + /*
> + * XXX pick_eevdf(cfs_rq) != se ?
> + */
> + if (pick_eevdf(cfs_rq) == pse)
> + goto preempt;
> +
> + return;
> + }
> +
> if (wakeup_preempt_entity(se, pse) == 1) {
> /*
> * Bias pick_next to pick the sched entity that is
> @@ -8256,7 +8490,7 @@ static void yield_task_fair(struct rq *r
>
> clear_buddies(cfs_rq, se);
>
> - if (curr->policy != SCHED_BATCH) {
> + if (sched_feat(EEVDF) || curr->policy != SCHED_BATCH) {
> update_rq_clock(rq);
> /*
> * Update run-time statistics of the 'current'.
> @@ -8269,6 +8503,8 @@ static void yield_task_fair(struct rq *r
> */
> rq_clock_skip_update(rq);
> }
> + if (sched_feat(EEVDF))
> + se->deadline += calc_delta_fair(se->slice, se);
>
> set_skip_buddy(se);
> }
> @@ -12012,8 +12248,8 @@ static void rq_offline_fair(struct rq *r
> static inline bool
> __entity_slice_used(struct sched_entity *se, int min_nr_tasks)
> {
> - u64 slice = sched_slice(cfs_rq_of(se), se);
> u64 rtime = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> + u64 slice = se->slice;
>
> return (rtime * min_nr_tasks > slice);
> }
> @@ -12728,7 +12964,7 @@ static unsigned int get_rr_interval_fair
> * idle runqueue:
> */
> if (rq->cfs.load.weight)
> - rr_interval = NS_TO_JIFFIES(sched_slice(cfs_rq_of(se), se));
> + rr_interval = NS_TO_JIFFIES(se->slice);
>
> return rr_interval;
> }
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -13,6 +13,7 @@ SCHED_FEAT(GENTLE_FAIR_SLEEPERS, true)
> * sleep+wake cycles. EEVDF placement strategy #1, #2 if disabled.
> */
> SCHED_FEAT(PLACE_LAG, true)
> +SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
>
> /*
> * Prefer to schedule the task we woke last (assuming it failed
> @@ -103,3 +104,5 @@ SCHED_FEAT(LATENCY_WARN, false)
>
> SCHED_FEAT(ALT_PERIOD, true)
> SCHED_FEAT(BASE_SLICE, true)
> +
> +SCHED_FEAT(EEVDF, true)
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3316,5 +3316,6 @@ static inline void switch_mm_cid(struct
> #endif
>
> extern u64 avg_vruntime(struct cfs_rq *cfs_rq);
> +extern int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se);
>
> #endif /* _KERNEL_SCHED_SCHED_H */
>
>
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