Messages in this thread Patch in this message | | | Date | Tue, 28 Mar 2023 11:26:28 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 06/17] sched/fair: Add lag based placement |
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With the introduction of avg_vruntime, it is possible to approximate lag (the entire purpose of introducing it in fact). Use this to do lag based placement over sleep+wake.
Specifically, the FAIR_SLEEPERS thing places things too far to the left and messes up the deadline aspect of EEVDF.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- include/linux/sched.h | 1 kernel/sched/core.c | 1 kernel/sched/fair.c | 129 ++++++++++++++++++++++++++++++++++-------------- kernel/sched/features.h | 8 ++ 4 files changed, 104 insertions(+), 35 deletions(-)
--- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -555,6 +555,7 @@ struct sched_entity { u64 sum_exec_runtime; u64 vruntime; u64 prev_sum_exec_runtime; + s64 vlag; u64 nr_migrations; --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4439,6 +4439,7 @@ static void __sched_fork(unsigned long c p->se.prev_sum_exec_runtime = 0; p->se.nr_migrations = 0; p->se.vruntime = 0; + p->se.vlag = 0; INIT_LIST_HEAD(&p->se.group_node); set_latency_offset(p); --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -689,6 +689,15 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq) return cfs_rq->min_vruntime + avg; } +/* + * lag_i = S - s_i = w_i * (V - v_i) + */ +void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) +{ + SCHED_WARN_ON(!se->on_rq); + se->vlag = avg_vruntime(cfs_rq) - se->vruntime; +} + static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime) { u64 min_vruntime = cfs_rq->min_vruntime; @@ -3417,6 +3426,8 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, unsigned long weight) { + unsigned long old_weight = se->load.weight; + if (se->on_rq) { /* commit outstanding execution time */ if (cfs_rq->curr == se) @@ -3429,6 +3440,14 @@ static void reweight_entity(struct cfs_r update_load_set(&se->load, weight); + if (!se->on_rq) { + /* + * Because we keep se->vlag = V - v_i, while: lag_i = w_i*(V - v), + * we need to scale se->vlag when w_i changes. + */ + se->vlag = div_s64(se->vlag * old_weight, weight); + } + #ifdef CONFIG_SMP do { u32 divider = get_pelt_divider(&se->avg); @@ -4778,49 +4797,86 @@ static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) { u64 vruntime = avg_vruntime(cfs_rq); + s64 lag = 0; - /* sleeps up to a single latency don't count. */ - if (!initial) { - unsigned long thresh; + /* + * Due to how V is constructed as the weighted average of entities, + * adding tasks with positive lag, or removing tasks with negative lag + * will move 'time' backwards, this can screw around with the lag of + * other tasks. + * + * EEVDF: placement strategy #1 / #2 + */ + if (sched_feat(PLACE_LAG) && cfs_rq->nr_running > 1) { + struct sched_entity *curr = cfs_rq->curr; + unsigned long load; - if (se_is_idle(se)) - thresh = sysctl_sched_min_granularity; - else - thresh = sysctl_sched_latency; + lag = se->vlag; /* - * Halve their sleep time's effect, to allow - * for a gentler effect of sleepers: + * If we want to place a task and preserve lag, we have to + * consider the effect of the new entity on the weighted + * average and compensate for this, otherwise lag can quickly + * evaporate: + * + * l_i = V - v_i <=> v_i = V - l_i + * + * V = v_avg = W*v_avg / W + * + * V' = (W*v_avg + w_i*v_i) / (W + w_i) + * = (W*v_avg + w_i(v_avg - l_i)) / (W + w_i) + * = v_avg + w_i*l_i/(W + w_i) + * + * l_i' = V' - v_i = v_avg + w_i*l_i/(W + w_i) - (v_avg - l) + * = l_i - w_i*l_i/(W + w_i) + * + * l_i = (W + w_i) * l_i' / W */ - if (sched_feat(GENTLE_FAIR_SLEEPERS)) - thresh >>= 1; + load = cfs_rq->avg_load; + if (curr && curr->on_rq) + load += curr->load.weight; + + lag *= load + se->load.weight; + if (WARN_ON_ONCE(!load)) + load = 1; + lag = div_s64(lag, load); - vruntime -= thresh; + vruntime -= lag; } - /* - * Pull vruntime of the entity being placed to the base level of - * cfs_rq, to prevent boosting it if placed backwards. - * However, min_vruntime can advance much faster than real time, with - * the extreme being when an entity with the minimal weight always runs - * on the cfs_rq. If the waking entity slept for a long time, its - * vruntime difference from min_vruntime may overflow s64 and their - * comparison may get inversed, so ignore the entity's original - * vruntime in that case. - * The maximal vruntime speedup is given by the ratio of normal to - * minimal weight: scale_load_down(NICE_0_LOAD) / MIN_SHARES. - * When placing a migrated waking entity, its exec_start has been set - * from a different rq. In order to take into account a possible - * divergence between new and prev rq's clocks task because of irq and - * stolen time, we take an additional margin. - * So, cutting off on the sleep time of - * 2^63 / scale_load_down(NICE_0_LOAD) ~ 104 days - * should be safe. - */ - if (entity_is_long_sleeper(se)) - se->vruntime = vruntime; - else - se->vruntime = max_vruntime(se->vruntime, vruntime); + if (sched_feat(FAIR_SLEEPERS)) { + + /* sleeps up to a single latency don't count. */ + if (!initial) { + unsigned long thresh; + + if (se_is_idle(se)) + thresh = sysctl_sched_min_granularity; + else + thresh = sysctl_sched_latency; + + /* + * Halve their sleep time's effect, to allow + * for a gentler effect of sleepers: + */ + if (sched_feat(GENTLE_FAIR_SLEEPERS)) + thresh >>= 1; + + vruntime -= thresh; + } + + /* + * Pull vruntime of the entity being placed to the base level of + * cfs_rq, to prevent boosting it if placed backwards. If the entity + * slept for a long time, don't even try to compare its vruntime with + * the base as it may be too far off and the comparison may get + * inversed due to s64 overflow. + */ + if (!entity_is_long_sleeper(se)) + vruntime = max_vruntime(se->vruntime, vruntime); + } + + se->vruntime = vruntime; } static void check_enqueue_throttle(struct cfs_rq *cfs_rq); @@ -4991,6 +5047,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, st clear_buddies(cfs_rq, se); + if (flags & DEQUEUE_SLEEP) + update_entity_lag(cfs_rq, se); + if (se != cfs_rq->curr) __dequeue_entity(cfs_rq, se); se->on_rq = 0; --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -1,12 +1,20 @@ /* SPDX-License-Identifier: GPL-2.0 */ + /* * Only give sleepers 50% of their service deficit. This allows * them to run sooner, but does not allow tons of sleepers to * rip the spread apart. */ +SCHED_FEAT(FAIR_SLEEPERS, false) SCHED_FEAT(GENTLE_FAIR_SLEEPERS, true) /* + * Using the avg_vruntime, do the right thing and preserve lag across + * sleep+wake cycles. EEVDF placement strategy #1, #2 if disabled. + */ +SCHED_FEAT(PLACE_LAG, true) + +/* * Prefer to schedule the task we woke last (assuming it failed * wakeup-preemption), since its likely going to consume data we * touched, increases cache locality.
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