Re: [PATCH 08/15] sched: Commit to EEVDF

[Joel Fernandes]

On Wed, May 31, 2023 at 01:58:47PM +0200, Peter Zijlstra wrote:
> EEVDF is a better defined scheduling policy, as a result it has less
> heuristics/tunables. There is no compelling reason to keep CFS around.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/debug.c    |    6 
>  kernel/sched/fair.c     |  465 +++---------------------------------------------

Whether EEVDF helps us improve our CFS latency issues or not, I do like the
merits of this diffstat alone and the lesser complexity and getting rid of
those horrible knobs is kinda nice.

For ChromeOS, we are experimenting with RT as well for our high priority
threads, and the PeterZ/Juri's patches on DL-server.

One of the issues is, on small number of CPU systems and overloaded CPUs, the
fair scheduler tends to eat up too much from high priority threads. That's
where RT kind of shines in our testing because it just tells those lower
priority CFS buggers to STFU.

I believe EEVDF will still have those issues (as its still a fair scheduler
if I'm not horribly mistaken).

But hey, less CFS knobs is only a good thing! And most people say fair.c is
one of the most complex beasts in the kernel so I'm all for this..

thanks,

 - Joel


>  kernel/sched/features.h |   12 -
>  kernel/sched/sched.h    |    5 
>  4 files changed, 38 insertions(+), 450 deletions(-)
> 
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -347,10 +347,7 @@ static __init int sched_init_debug(void)
>  	debugfs_create_file("preempt", 0644, debugfs_sched, NULL, &sched_dynamic_fops);
>  #endif
>  
> -	debugfs_create_u32("latency_ns", 0644, debugfs_sched, &sysctl_sched_latency);
>  	debugfs_create_u32("min_granularity_ns", 0644, debugfs_sched, &sysctl_sched_min_granularity);
> -	debugfs_create_u32("idle_min_granularity_ns", 0644, debugfs_sched, &sysctl_sched_idle_min_granularity);
> -	debugfs_create_u32("wakeup_granularity_ns", 0644, debugfs_sched, &sysctl_sched_wakeup_granularity);
>  
>  	debugfs_create_u32("latency_warn_ms", 0644, debugfs_sched, &sysctl_resched_latency_warn_ms);
>  	debugfs_create_u32("latency_warn_once", 0644, debugfs_sched, &sysctl_resched_latency_warn_once);
> @@ -865,10 +862,7 @@ static void sched_debug_header(struct se
>  	SEQ_printf(m, "  .%-40s: %Ld\n", #x, (long long)(x))
>  #define PN(x) \
>  	SEQ_printf(m, "  .%-40s: %Ld.%06ld\n", #x, SPLIT_NS(x))
> -	PN(sysctl_sched_latency);
>  	PN(sysctl_sched_min_granularity);
> -	PN(sysctl_sched_idle_min_granularity);
> -	PN(sysctl_sched_wakeup_granularity);
>  	P(sysctl_sched_child_runs_first);
>  	P(sysctl_sched_features);
>  #undef PN
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -58,22 +58,6 @@
>  #include "autogroup.h"
>  
>  /*
> - * Targeted preemption latency for CPU-bound tasks:
> - *
> - * NOTE: this latency value is not the same as the concept of
> - * 'timeslice length' - timeslices in CFS are of variable length
> - * and have no persistent notion like in traditional, time-slice
> - * based scheduling concepts.
> - *
> - * (to see the precise effective timeslice length of your workload,
> - *  run vmstat and monitor the context-switches (cs) field)
> - *
> - * (default: 6ms * (1 + ilog(ncpus)), units: nanoseconds)
> - */
> -unsigned int sysctl_sched_latency			= 6000000ULL;
> -static unsigned int normalized_sysctl_sched_latency	= 6000000ULL;
> -
> -/*
>   * The initial- and re-scaling of tunables is configurable
>   *
>   * Options are:
> @@ -95,36 +79,11 @@ unsigned int sysctl_sched_min_granularit
>  static unsigned int normalized_sysctl_sched_min_granularity	= 750000ULL;
>  
>  /*
> - * Minimal preemption granularity for CPU-bound SCHED_IDLE tasks.
> - * Applies only when SCHED_IDLE tasks compete with normal tasks.
> - *
> - * (default: 0.75 msec)
> - */
> -unsigned int sysctl_sched_idle_min_granularity			= 750000ULL;
> -
> -/*
> - * This value is kept at sysctl_sched_latency/sysctl_sched_min_granularity
> - */
> -static unsigned int sched_nr_latency = 8;
> -
> -/*
>   * After fork, child runs first. If set to 0 (default) then
>   * parent will (try to) run first.
>   */
>  unsigned int sysctl_sched_child_runs_first __read_mostly;
>  
> -/*
> - * SCHED_OTHER wake-up granularity.
> - *
> - * This option delays the preemption effects of decoupled workloads
> - * and reduces their over-scheduling. Synchronous workloads will still
> - * have immediate wakeup/sleep latencies.
> - *
> - * (default: 1 msec * (1 + ilog(ncpus)), units: nanoseconds)
> - */
> -unsigned int sysctl_sched_wakeup_granularity			= 1000000UL;
> -static unsigned int normalized_sysctl_sched_wakeup_granularity	= 1000000UL;
> -
>  const_debug unsigned int sysctl_sched_migration_cost	= 500000UL;
>  
>  int sched_thermal_decay_shift;
> @@ -279,8 +238,6 @@ static void update_sysctl(void)
>  #define SET_SYSCTL(name) \
>  	(sysctl_##name = (factor) * normalized_sysctl_##name)
>  	SET_SYSCTL(sched_min_granularity);
> -	SET_SYSCTL(sched_latency);
> -	SET_SYSCTL(sched_wakeup_granularity);
>  #undef SET_SYSCTL
>  }
>  
> @@ -888,30 +845,6 @@ struct sched_entity *__pick_first_entity
>  	return __node_2_se(left);
>  }
>  
> -static struct sched_entity *__pick_next_entity(struct sched_entity *se)
> -{
> -	struct rb_node *next = rb_next(&se->run_node);
> -
> -	if (!next)
> -		return NULL;
> -
> -	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
>   *
> @@ -1008,85 +941,15 @@ int sched_update_scaling(void)
>  {
>  	unsigned int factor = get_update_sysctl_factor();
>  
> -	sched_nr_latency = DIV_ROUND_UP(sysctl_sched_latency,
> -					sysctl_sched_min_granularity);
> -
>  #define WRT_SYSCTL(name) \
>  	(normalized_sysctl_##name = sysctl_##name / (factor))
>  	WRT_SYSCTL(sched_min_granularity);
> -	WRT_SYSCTL(sched_latency);
> -	WRT_SYSCTL(sched_wakeup_granularity);
>  #undef WRT_SYSCTL
>  
>  	return 0;
>  }
>  #endif
>  
> -/*
> - * 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
> - * this period because otherwise the slices get too small.
> - *
> - * p = (nr <= nl) ? l : l*nr/nl
> - */
> -static u64 __sched_period(unsigned long nr_running)
> -{
> -	if (unlikely(nr_running > sched_nr_latency))
> -		return nr_running * sysctl_sched_min_granularity;
> -	else
> -		return sysctl_sched_latency;
> -}
> -
> -static bool sched_idle_cfs_rq(struct cfs_rq *cfs_rq);
> -
> -/*
> - * We calculate the wall-time slice from the period by taking a part
> - * proportional to the weight.
> - *
> - * s = p*P[w/rw]
> - */
> -static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> -{
> -	unsigned int nr_running = cfs_rq->nr_running;
> -	struct sched_entity *init_se = se;
> -	unsigned int min_gran;
> -	u64 slice;
> -
> -	if (sched_feat(ALT_PERIOD))
> -		nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
> -
> -	slice = __sched_period(nr_running + !se->on_rq);
> -
> -	for_each_sched_entity(se) {
> -		struct load_weight *load;
> -		struct load_weight lw;
> -		struct cfs_rq *qcfs_rq;
> -
> -		qcfs_rq = cfs_rq_of(se);
> -		load = &qcfs_rq->load;
> -
> -		if (unlikely(!se->on_rq)) {
> -			lw = qcfs_rq->load;
> -
> -			update_load_add(&lw, se->load.weight);
> -			load = &lw;
> -		}
> -		slice = __calc_delta(slice, se->load.weight, load);
> -	}
> -
> -	if (sched_feat(BASE_SLICE)) {
> -		if (se_is_idle(init_se) && !sched_idle_cfs_rq(cfs_rq))
> -			min_gran = sysctl_sched_idle_min_granularity;
> -		else
> -			min_gran = sysctl_sched_min_granularity;
> -
> -		slice = max_t(u64, slice, min_gran);
> -	}
> -
> -	return slice;
> -}
> -
>  static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se);
>  
>  /*
> @@ -1098,35 +961,25 @@ static void update_deadline(struct cfs_r
>  	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
> -		 * sysctl_sched_min_granularity.
> -		 */
> -		se->slice = sysctl_sched_min_granularity;
> -
> -		/*
> -		 * The task has consumed its request, reschedule.
> -		 */
> -		if (cfs_rq->nr_running > 1) {
> -			resched_curr(rq_of(cfs_rq));
> -			clear_buddies(cfs_rq, se);
> -		}
> -	} 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);
> -	}
> +	/*
> +	 * For EEVDF the virtual time slope is determined by w_i (iow.
> +	 * nice) while the request time r_i is determined by
> +	 * sysctl_sched_min_granularity.
> +	 */
> +	se->slice = sysctl_sched_min_granularity;
>  
>  	/*
>  	 * EEVDF: vd_i = ve_i + r_i / w_i
>  	 */
>  	se->deadline = se->vruntime + calc_delta_fair(se->slice, se);
> +
> +	/*
> +	 * The task has consumed its request, reschedule.
> +	 */
> +	if (cfs_rq->nr_running > 1) {
> +		resched_curr(rq_of(cfs_rq));
> +		clear_buddies(cfs_rq, se);
> +	}
>  }
>  
>  #include "pelt.h"
> @@ -5055,19 +4908,6 @@ static inline void update_misfit_status(
>  
>  #endif /* CONFIG_SMP */
>  
> -static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
> -{
> -#ifdef CONFIG_SCHED_DEBUG
> -	s64 d = se->vruntime - cfs_rq->min_vruntime;
> -
> -	if (d < 0)
> -		d = -d;
> -
> -	if (d > 3*sysctl_sched_latency)
> -		schedstat_inc(cfs_rq->nr_spread_over);
> -#endif
> -}
> -
>  static void
>  place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>  {
> @@ -5218,7 +5058,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
>  
>  	check_schedstat_required();
>  	update_stats_enqueue_fair(cfs_rq, se, flags);
> -	check_spread(cfs_rq, se);
>  	if (!curr)
>  		__enqueue_entity(cfs_rq, se);
>  	se->on_rq = 1;
> @@ -5230,17 +5069,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
>  	}
>  }
>  
> -static void __clear_buddies_last(struct sched_entity *se)
> -{
> -	for_each_sched_entity(se) {
> -		struct cfs_rq *cfs_rq = cfs_rq_of(se);
> -		if (cfs_rq->last != se)
> -			break;
> -
> -		cfs_rq->last = NULL;
> -	}
> -}
> -
>  static void __clear_buddies_next(struct sched_entity *se)
>  {
>  	for_each_sched_entity(se) {
> @@ -5252,27 +5080,10 @@ static void __clear_buddies_next(struct
>  	}
>  }
>  
> -static void __clear_buddies_skip(struct sched_entity *se)
> -{
> -	for_each_sched_entity(se) {
> -		struct cfs_rq *cfs_rq = cfs_rq_of(se);
> -		if (cfs_rq->skip != se)
> -			break;
> -
> -		cfs_rq->skip = NULL;
> -	}
> -}
> -
>  static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -	if (cfs_rq->last == se)
> -		__clear_buddies_last(se);
> -
>  	if (cfs_rq->next == se)
>  		__clear_buddies_next(se);
> -
> -	if (cfs_rq->skip == se)
> -		__clear_buddies_skip(se);
>  }
>  
>  static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
> @@ -5330,45 +5141,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
>  		update_idle_cfs_rq_clock_pelt(cfs_rq);
>  }
>  
> -/*
> - * Preempt the current task with a newly woken task if needed:
> - */
> -static void
> -check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> -{
> -	unsigned long delta_exec;
> -	struct sched_entity *se;
> -	s64 delta;
> -
> -	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> -	if (delta_exec > curr->slice) {
> -		resched_curr(rq_of(cfs_rq));
> -		/*
> -		 * The current task ran long enough, ensure it doesn't get
> -		 * re-elected due to buddy favours.
> -		 */
> -		clear_buddies(cfs_rq, curr);
> -		return;
> -	}
> -
> -	/*
> -	 * Ensure that a task that missed wakeup preemption by a
> -	 * narrow margin doesn't have to wait for a full slice.
> -	 * This also mitigates buddy induced latencies under load.
> -	 */
> -	if (delta_exec < sysctl_sched_min_granularity)
> -		return;
> -
> -	se = __pick_first_entity(cfs_rq);
> -	delta = curr->vruntime - se->vruntime;
> -
> -	if (delta < 0)
> -		return;
> -
> -	if (delta > curr->slice)
> -		resched_curr(rq_of(cfs_rq));
> -}
> -
>  static void
>  set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> @@ -5407,9 +5179,6 @@ set_next_entity(struct cfs_rq *cfs_rq, s
>  	se->prev_sum_exec_runtime = se->sum_exec_runtime;
>  }
>  
> -static int
> -wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
> -
>  /*
>   * Pick the next process, keeping these things in mind, in this order:
>   * 1) keep things fair between processes/task groups
> @@ -5420,53 +5189,14 @@ 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, *se;
> -
> -	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 = pick_cfs(cfs_rq, curr);
> -
>  	/*
> -	 * Avoid running the skip buddy, if running something else can
> -	 * be done without getting too unfair.
> +	 * Enabling NEXT_BUDDY will affect latency but not fairness.
>  	 */
> -	if (cfs_rq->skip && cfs_rq->skip == se) {
> -		struct sched_entity *second;
> +	if (sched_feat(NEXT_BUDDY) &&
> +	    cfs_rq->next && entity_eligible(cfs_rq, cfs_rq->next))
> +		return cfs_rq->next;
>  
> -		if (se == curr) {
> -			second = __pick_first_entity(cfs_rq);
> -		} else {
> -			second = __pick_next_entity(se);
> -			if (!second || (curr && entity_before(curr, second)))
> -				second = curr;
> -		}
> -
> -		if (second && wakeup_preempt_entity(second, left) < 1)
> -			se = second;
> -	}
> -
> -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> -		/*
> -		 * Someone really wants this to run. If it's not unfair, run it.
> -		 */
> -		se = cfs_rq->next;
> -	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
> -		/*
> -		 * Prefer last buddy, try to return the CPU to a preempted task.
> -		 */
> -		se = cfs_rq->last;
> -	}
> -
> -	return se;
> +	return pick_eevdf(cfs_rq);
>  }
>  
>  static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq);
> @@ -5483,8 +5213,6 @@ static void put_prev_entity(struct cfs_r
>  	/* throttle cfs_rqs exceeding runtime */
>  	check_cfs_rq_runtime(cfs_rq);
>  
> -	check_spread(cfs_rq, prev);
> -
>  	if (prev->on_rq) {
>  		update_stats_wait_start_fair(cfs_rq, prev);
>  		/* Put 'current' back into the tree. */
> @@ -5525,9 +5253,6 @@ entity_tick(struct cfs_rq *cfs_rq, struc
>  			hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
>  		return;
>  #endif
> -
> -	if (!sched_feat(EEVDF) && cfs_rq->nr_running > 1)
> -		check_preempt_tick(cfs_rq, curr);
>  }
>  
>  
> @@ -6561,8 +6286,7 @@ static void hrtick_update(struct rq *rq)
>  	if (!hrtick_enabled_fair(rq) || curr->sched_class != &fair_sched_class)
>  		return;
>  
> -	if (cfs_rq_of(&curr->se)->nr_running < sched_nr_latency)
> -		hrtick_start_fair(rq, curr);
> +	hrtick_start_fair(rq, curr);
>  }
>  #else /* !CONFIG_SCHED_HRTICK */
>  static inline void
> @@ -6603,17 +6327,6 @@ static int sched_idle_rq(struct rq *rq)
>  			rq->nr_running);
>  }
>  
> -/*
> - * Returns true if cfs_rq only has SCHED_IDLE entities enqueued. Note the use
> - * of idle_nr_running, which does not consider idle descendants of normal
> - * entities.
> - */
> -static bool sched_idle_cfs_rq(struct cfs_rq *cfs_rq)
> -{
> -	return cfs_rq->nr_running &&
> -		cfs_rq->nr_running == cfs_rq->idle_nr_running;
> -}
> -
>  #ifdef CONFIG_SMP
>  static int sched_idle_cpu(int cpu)
>  {
> @@ -8099,66 +7812,6 @@ balance_fair(struct rq *rq, struct task_
>  }
>  #endif /* CONFIG_SMP */
>  
> -static unsigned long wakeup_gran(struct sched_entity *se)
> -{
> -	unsigned long gran = sysctl_sched_wakeup_granularity;
> -
> -	/*
> -	 * Since its curr running now, convert the gran from real-time
> -	 * to virtual-time in his units.
> -	 *
> -	 * By using 'se' instead of 'curr' we penalize light tasks, so
> -	 * they get preempted easier. That is, if 'se' < 'curr' then
> -	 * the resulting gran will be larger, therefore penalizing the
> -	 * lighter, if otoh 'se' > 'curr' then the resulting gran will
> -	 * be smaller, again penalizing the lighter task.
> -	 *
> -	 * This is especially important for buddies when the leftmost
> -	 * task is higher priority than the buddy.
> -	 */
> -	return calc_delta_fair(gran, se);
> -}
> -
> -/*
> - * Should 'se' preempt 'curr'.
> - *
> - *             |s1
> - *        |s2
> - *   |s3
> - *         g
> - *      |<--->|c
> - *
> - *  w(c, s1) = -1
> - *  w(c, s2) =  0
> - *  w(c, s3) =  1
> - *
> - */
> -static int
> -wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> -{
> -	s64 gran, vdiff = curr->vruntime - se->vruntime;
> -
> -	if (vdiff <= 0)
> -		return -1;
> -
> -	gran = wakeup_gran(se);
> -	if (vdiff > gran)
> -		return 1;
> -
> -	return 0;
> -}
> -
> -static void set_last_buddy(struct sched_entity *se)
> -{
> -	for_each_sched_entity(se) {
> -		if (SCHED_WARN_ON(!se->on_rq))
> -			return;
> -		if (se_is_idle(se))
> -			return;
> -		cfs_rq_of(se)->last = se;
> -	}
> -}
> -
>  static void set_next_buddy(struct sched_entity *se)
>  {
>  	for_each_sched_entity(se) {
> @@ -8170,12 +7823,6 @@ static void set_next_buddy(struct sched_
>  	}
>  }
>  
> -static void set_skip_buddy(struct sched_entity *se)
> -{
> -	for_each_sched_entity(se)
> -		cfs_rq_of(se)->skip = se;
> -}
> -
>  /*
>   * Preempt the current task with a newly woken task if needed:
>   */
> @@ -8184,7 +7831,6 @@ static void check_preempt_wakeup(struct
>  	struct task_struct *curr = rq->curr;
>  	struct sched_entity *se = &curr->se, *pse = &p->se;
>  	struct cfs_rq *cfs_rq = task_cfs_rq(curr);
> -	int scale = cfs_rq->nr_running >= sched_nr_latency;
>  	int next_buddy_marked = 0;
>  	int cse_is_idle, pse_is_idle;
>  
> @@ -8200,7 +7846,7 @@ static void check_preempt_wakeup(struct
>  	if (unlikely(throttled_hierarchy(cfs_rq_of(pse))))
>  		return;
>  
> -	if (sched_feat(NEXT_BUDDY) && scale && !(wake_flags & WF_FORK)) {
> +	if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK)) {
>  		set_next_buddy(pse);
>  		next_buddy_marked = 1;
>  	}
> @@ -8248,44 +7894,16 @@ static void check_preempt_wakeup(struct
>  	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
> -		 * triggering this preemption.
> -		 */
> -		if (!next_buddy_marked)
> -			set_next_buddy(pse);
> +	/*
> +	 * XXX pick_eevdf(cfs_rq) != se ?
> +	 */
> +	if (pick_eevdf(cfs_rq) == pse)
>  		goto preempt;
> -	}
>  
>  	return;
>  
>  preempt:
>  	resched_curr(rq);
> -	/*
> -	 * Only set the backward buddy when the current task is still
> -	 * on the rq. This can happen when a wakeup gets interleaved
> -	 * with schedule on the ->pre_schedule() or idle_balance()
> -	 * point, either of which can * drop the rq lock.
> -	 *
> -	 * Also, during early boot the idle thread is in the fair class,
> -	 * for obvious reasons its a bad idea to schedule back to it.
> -	 */
> -	if (unlikely(!se->on_rq || curr == rq->idle))
> -		return;
> -
> -	if (sched_feat(LAST_BUDDY) && scale && entity_is_task(se))
> -		set_last_buddy(se);
>  }
>  
>  #ifdef CONFIG_SMP
> @@ -8486,8 +8104,6 @@ static void put_prev_task_fair(struct rq
>  
>  /*
>   * sched_yield() is very simple
> - *
> - * The magic of dealing with the ->skip buddy is in pick_next_entity.
>   */
>  static void yield_task_fair(struct rq *rq)
>  {
> @@ -8503,23 +8119,19 @@ static void yield_task_fair(struct rq *r
>  
>  	clear_buddies(cfs_rq, se);
>  
> -	if (sched_feat(EEVDF) || curr->policy != SCHED_BATCH) {
> -		update_rq_clock(rq);
> -		/*
> -		 * Update run-time statistics of the 'current'.
> -		 */
> -		update_curr(cfs_rq);
> -		/*
> -		 * Tell update_rq_clock() that we've just updated,
> -		 * so we don't do microscopic update in schedule()
> -		 * and double the fastpath cost.
> -		 */
> -		rq_clock_skip_update(rq);
> -	}
> -	if (sched_feat(EEVDF))
> -		se->deadline += calc_delta_fair(se->slice, se);
> +	update_rq_clock(rq);
> +	/*
> +	 * Update run-time statistics of the 'current'.
> +	 */
> +	update_curr(cfs_rq);
> +	/*
> +	 * Tell update_rq_clock() that we've just updated,
> +	 * so we don't do microscopic update in schedule()
> +	 * and double the fastpath cost.
> +	 */
> +	rq_clock_skip_update(rq);
>  
> -	set_skip_buddy(se);
> +	se->deadline += calc_delta_fair(se->slice, se);
>  }
>  
>  static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)
> @@ -8762,8 +8374,7 @@ static int task_hot(struct task_struct *
>  	 * Buddy candidates are cache hot:
>  	 */
>  	if (sched_feat(CACHE_HOT_BUDDY) && env->dst_rq->nr_running &&
> -			(&p->se == cfs_rq_of(&p->se)->next ||
> -			 &p->se == cfs_rq_of(&p->se)->last))
> +	    (&p->se == cfs_rq_of(&p->se)->next))
>  		return 1;
>  
>  	if (sysctl_sched_migration_cost == -1)
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -15,13 +15,6 @@ SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
>  SCHED_FEAT(NEXT_BUDDY, false)
>  
>  /*
> - * Prefer to schedule the task that ran last (when we did
> - * wake-preempt) as that likely will touch the same data, increases
> - * cache locality.
> - */
> -SCHED_FEAT(LAST_BUDDY, true)
> -
> -/*
>   * Consider buddies to be cache hot, decreases the likeliness of a
>   * cache buddy being migrated away, increases cache locality.
>   */
> @@ -93,8 +86,3 @@ SCHED_FEAT(UTIL_EST, true)
>  SCHED_FEAT(UTIL_EST_FASTUP, true)
>  
>  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
> @@ -576,8 +576,6 @@ struct cfs_rq {
>  	 */
>  	struct sched_entity	*curr;
>  	struct sched_entity	*next;
> -	struct sched_entity	*last;
> -	struct sched_entity	*skip;
>  
>  #ifdef	CONFIG_SCHED_DEBUG
>  	unsigned int		nr_spread_over;
> @@ -2484,9 +2482,6 @@ extern const_debug unsigned int sysctl_s
>  extern unsigned int sysctl_sched_min_granularity;
>  
>  #ifdef CONFIG_SCHED_DEBUG
> -extern unsigned int sysctl_sched_latency;
> -extern unsigned int sysctl_sched_idle_min_granularity;
> -extern unsigned int sysctl_sched_wakeup_granularity;
>  extern int sysctl_resched_latency_warn_ms;
>  extern int sysctl_resched_latency_warn_once;
>  
> 
>