Re: [PATCH v12 8/8] sched/fair: Add latency list

From: Vincent Guittot
Date: Thu Mar 02 2023 - 02:50:59 EST

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In reply to: shrikanth hegde: "Re: [PATCH v12 8/8] sched/fair: Add latency list"
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On Wed, 1 Mar 2023 at 19:48, shrikanth hegde <sshegde@xxxxxxxxxxxxxxxxxx> wrote:
>
>
> On 2/24/23 3:04 PM, Vincent Guittot wrote:
> > Add a rb tree for latency sensitive entities so we can schedule the most
> > sensitive one first even when it failed to preempt current at wakeup or
> > when it got quickly preempted by another entity of higher priority.
> >
> > In order to keep fairness, the latency is used once at wakeup to get a
> > minimum slice and not during the following scheduling slice to prevent
> > long running entity to got more running time than allocated to his nice
> > priority.
> >
> > The rb tree enables to cover the last corner case where latency
> > sensitive entity can't got schedule quickly after the wakeup.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > ---
> > include/linux/sched.h | 1 +
> > kernel/sched/core.c | 1 +
> > kernel/sched/fair.c | 109 ++++++++++++++++++++++++++++++++++++++++--
> > kernel/sched/sched.h | 1 +
> > 4 files changed, 109 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 38decae3e156..41bb92be5ecc 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -548,6 +548,7 @@ struct sched_entity {
> > /* For load-balancing: */
> > struct load_weight load;
> > struct rb_node run_node;
> > + struct rb_node latency_node;
>
> Ran pahole to see if the frequently accessed fields change across cachelines.
> There is no change in cachelines of task_struct, whereas sched_entity differs
> due to latency_node. Maybe the latency_node could be placed after
> runnable_weight as there is space available in that cacheline.

I will run some test on my system to confimr your results but we can
move latency_node field if it helps cache hit stats

>
>
> 6.2
> #pahole sched_entity
> struct sched_entity {
> struct load_weight load; /* 0 16 */
>
> /* XXX last struct has 4 bytes of padding */
>
> struct rb_node run_node; /* 16 24 */
> struct list_head group_node; /* 40 16 */
> unsigned int on_rq; /* 56 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> u64 exec_start; /* 64 8 */
> u64 sum_exec_runtime; /* 72 8 */
> u64 vruntime; /* 80 8 */
> u64 prev_sum_exec_runtime; /* 88 8 */
> u64 nr_migrations; /* 96 8 */
> int depth; /* 104 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct sched_entity * parent; /* 112 8 */
> struct cfs_rq * cfs_rq; /* 120 8 */
> /* --- cacheline 1 boundary (128 bytes) --- */
> struct cfs_rq * my_q; /* 128 8 */
> long unsigned int runnable_weight; /* 136 8 */
>
> /* XXX 112 bytes hole, try to pack */
>
> /* --- cacheline 2 boundary (256 bytes) --- */
> struct sched_avg avg; /* 256 128 */
>
>
> 6.2 + V12 patch
> #pahole sched_entity
> struct sched_entity {
> struct load_weight load; /* 0 16 */
>
> /* XXX last struct has 4 bytes of padding */
>
> struct rb_node run_node; /* 16 24 */
> struct rb_node latency_node; /* 40 24 */
> struct list_head group_node; /* 64 16 */
> unsigned int on_rq; /* 80 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> u64 exec_start; /* 88 8 */
> u64 sum_exec_runtime; /* 96 8 */
> u64 vruntime; /* 104 8 */
> u64 prev_sum_exec_runtime; /* 112 8 */
> u64 nr_migrations; /* 120 8 */
> /* --- cacheline 1 boundary (128 bytes) --- */
> int depth; /* 128 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct sched_entity * parent; /* 136 8 */
> struct cfs_rq * cfs_rq; /* 144 8 */
> struct cfs_rq * my_q; /* 152 8 */
>
>
> 6.2 + V12 patch + Re-shuffle of latency_node
> #pahole sched_entity
> struct sched_entity {
> struct load_weight load; /* 0 16 */
>
> /* XXX last struct has 4 bytes of padding */
>
> struct rb_node run_node; /* 16 24 */
> struct list_head group_node; /* 40 16 */
> unsigned int on_rq; /* 56 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> u64 exec_start; /* 64 8 */
> u64 sum_exec_runtime; /* 72 8 */
> u64 vruntime; /* 80 8 */
> u64 prev_sum_exec_runtime; /* 88 8 */
> u64 nr_migrations; /* 96 8 */
> int depth; /* 104 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct sched_entity * parent; /* 112 8 */
> struct cfs_rq * cfs_rq; /* 120 8 */
> /* --- cacheline 1 boundary (128 bytes) --- */
> struct cfs_rq * my_q; /* 128 8 */
> long unsigned int runnable_weight; /* 136 8 */
> struct rb_node latency_node; /* 144 24 */
> long int latency_offset; /* 168 8 */
>
> /* XXX 80 bytes hole, try to pack */
>
> /* --- cacheline 2 boundary (256 bytes) --- */
>
>
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a2b52cf5e1bb..1e93aaaeead2 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -548,7 +548,6 @@ struct sched_entity {
> /* For load-balancing: */
> struct load_weight load;
> struct rb_node run_node;
> - struct rb_node latency_node;
> struct list_head group_node;
> unsigned int on_rq;
>
> @@ -569,6 +568,7 @@ struct sched_entity {
> /* cached value of my_q->h_nr_running */
> unsigned long runnable_weight;
> #endif
> + struct rb_node latency_node;
> /* preemption offset in ns */
> long latency_offset;
>
>
> Ran the schbench and hackbench with this patch series. Here comparison is
> between 6.2 stable tree, 6.2 + Patch and 6.2 + patch + above re-arrange of
> latency_node. Ran two cgroups, in one cgroup running stress-ng at 50%(group1)
> and other is running these benchmarks (group2). Set the latency nice
> of group2 to -20. These are run on Power system with 12 cores with SMT=8.
> Total of 96 CPU.
>
> schbench gets lower latency compared to stabletree. Whereas hackbench seems
> to regress under this case. Maybe i am doing something wrong. I will re-run
> and attach the numbers to series.
> Please suggest if any variation in the test i need to try.

hackbench takes advanatge of a latency nice 19 as it mainly wants to
run longer slice to move forward rather than preempting others all the
time

>
> Re-arrange seems to help the patch series by avoiding an cacheline miss.
>
> =========================
> schbench
> =========================
> 6.2 | 6.2 + V12 | 6.2 + V12 + re-arrange
> 1 Thread
> 50.0th: 9.00 | 9.00 | 9.50
> 75.0th: 10.50 | 10.00 | 9.50
> 90.0th: 11.00 | 11.00 | 10.50
> 95.0th: 11.00 | 11.00 | 11.00
> 99.0th: 11.50 | 11.50 | 11.50
> 99.5th: 12.50 | 12.00 | 12.00
> 99.9th: 14.50 | 13.50 | 12.00
> 2 Threads
> 50.0th: 9.50 | 9.50 | 8.50
> 75.0th: 11.00 | 10.50 | 9.50
> 90.0th: 13.50 | 11.50 | 10.50
> 95.0th: 14.00 | 12.00 | 11.00
> 99.0th: 15.50 | 13.50 | 12.00
> 99.5th: 16.00 | 14.00 | 12.00
> 99.9th: 17.00 | 16.00 | 16.50
> 4 Threads
> 50.0th: 11.50 | 11.50 | 10.50
> 75.0th: 13.50 | 12.50 | 12.50
> 90.0th: 15.50 | 14.50 | 14.00
> 95.0th: 16.50 | 15.50 | 14.50
> 99.0th: 20.00 | 17.50 | 16.50
> 99.5th: 20.50 | 18.50 | 17.00
> 99.9th: 22.50 | 21.00 | 19.00
> 8 Threads
> 50.0th: 14.00 | 14.00 | 14.00
> 75.0th: 16.00 | 16.00 | 16.00
> 90.0th: 18.00 | 18.00 | 17.50
> 95.0th: 18.50 | 18.50 | 18.50
> 99.0th: 20.00 | 20.00 | 20.00
> 99.5th: 20.50 | 21.50 | 21.00
> 99.9th: 22.50 | 23.50 | 23.00
> 16 Threads
> 50.0th: 19.00 | 18.50 | 19.00
> 75.0th: 23.00 | 22.50 | 23.00
> 90.0th: 25.00 | 25.50 | 25.00
> 95.0th: 26.50 | 26.50 | 26.00
> 99.0th: 28.50 | 29.00 | 28.50
> 99.5th: 31.00 | 30.00 | 30.00
> 99.9th: 5626.00 | 4761.50 | 32.50
> 32 Threads
> 50.0th: 27.00 | 27.50 | 29.00
> 75.0th: 35.50 | 36.50 | 38.50
> 90.0th: 42.00 | 44.00 | 50.50
> 95.0th: 447.50 | 2959.00 | 8544.00
> 99.0th: 7372.00 | 17032.00 | 19136.00
> 99.5th: 15360.00 | 19808.00 | 20704.00
> 99.9th: 20640.00 | 30048.00 | 30048.00
>
> ====================
> hackbench
> ====================
> 6.2 | 6.2 + V12 | 6.2+ V12 +re-arrange
>
> Process 10 Time: 0.35 | 0.42 | 0.41
> Process 20 Time: 0.61 | 0.76 | 0.76
> Process 30 Time: 0.87 | 1.06 | 1.05
> thread 10 Time: 0.35 | 0.43 | 0.42
> thread 20 Time: 0.66 | 0.79 | 0.78
> Process(Pipe) 10 Time: 0.21 | 0.33 | 0.32
> Process(Pipe) 20 Time: 0.34 | 0.52 | 0.52
> Process(Pipe) 30 Time: 0.46 | 0.72 | 0.71
> thread(Pipe) 10 Time: 0.21 | 0.34 | 0.34
> thread(Pipe) 20 Time: 0.36 | 0.56 | 0.56
>
>
> > struct list_head group_node;
> > unsigned int on_rq;
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 093cc1af73dc..752fd364216c 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4434,6 +4434,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
> > p->se.nr_migrations = 0;
> > p->se.vruntime = 0;
> > INIT_LIST_HEAD(&p->se.group_node);
> > + RB_CLEAR_NODE(&p->se.latency_node);
> >
> > #ifdef CONFIG_FAIR_GROUP_SCHED
> > p->se.cfs_rq = NULL;
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 125a6ff53378..e2aeb4511686 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -680,7 +680,85 @@ struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
> >
> > return __node_2_se(last);
> > }
> > +#endif
> >
> > +/**************************************************************
> > + * Scheduling class tree data structure manipulation methods:
> > + * for latency
> > + */
> > +
> > +static inline bool latency_before(struct sched_entity *a,
> > + struct sched_entity *b)
> > +{
> > + return (s64)(a->vruntime + a->latency_offset - b->vruntime - b->latency_offset) < 0;
> > +}
> > +
> > +#define __latency_node_2_se(node) \
> > + rb_entry((node), struct sched_entity, latency_node)
> > +
> > +static inline bool __latency_less(struct rb_node *a, const struct rb_node *b)
> > +{
> > + return latency_before(__latency_node_2_se(a), __latency_node_2_se(b));
> > +}
> > +
> > +/*
> > + * Enqueue an entity into the latency rb-tree:
> > + */
> > +static void __enqueue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> > +{
> > +
> > + /* Only latency sensitive entity can be added to the list */
> > + if (se->latency_offset >= 0)
> > + return;
> > +
> > + if (!RB_EMPTY_NODE(&se->latency_node))
> > + return;
> > +
> > + /*
> > + * The entity is always added the latency list at wakeup.
> > + * Then, a not waking up entity that is put back in the list after an
> > + * execution time less than sysctl_sched_min_granularity, means that
> > + * the entity has been preempted by a higher sched class or an entity
> > + * with higher latency constraint. In thi case, the entity is also put
> > + * back in the latency list so it gets a chance to run 1st during the
> > + * next slice.
> > + */
> > + if (!(flags & ENQUEUE_WAKEUP)) {
> > + u64 delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> > +
> > + if (delta_exec >= sysctl_sched_min_granularity)
> > + return;
> > + }
> > +
> > + rb_add_cached(&se->latency_node, &cfs_rq->latency_timeline, __latency_less);
> > +}
> > +
> > +/*
> > + * Dequeue an entity from the latency rb-tree and return true if it was really
> > + * part of the rb-tree:
> > + */
> > +static bool __dequeue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > +{
> > + if (!RB_EMPTY_NODE(&se->latency_node)) {
> > + rb_erase_cached(&se->latency_node, &cfs_rq->latency_timeline);
> > + RB_CLEAR_NODE(&se->latency_node);
> > + return true;
> > + }
> > +
> > + return false;
> > +}
> > +
> > +static struct sched_entity *__pick_first_latency(struct cfs_rq *cfs_rq)
> > +{
> > + struct rb_node *left = rb_first_cached(&cfs_rq->latency_timeline);
> > +
> > + if (!left)
> > + return NULL;
> > +
> > + return __latency_node_2_se(left);
> > +}
> > +
> > +#ifdef CONFIG_SCHED_DEBUG
> > /**************************************************************
> > * Scheduling class statistics methods:
> > */
> > @@ -4758,8 +4836,10 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> > check_schedstat_required();
> > update_stats_enqueue_fair(cfs_rq, se, flags);
> > check_spread(cfs_rq, se);
> > - if (!curr)
> > + if (!curr) {
> > __enqueue_entity(cfs_rq, se);
> > + __enqueue_latency(cfs_rq, se, flags);
> > + }
> > se->on_rq = 1;
> >
> > if (cfs_rq->nr_running == 1) {
> > @@ -4845,8 +4925,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >
> > clear_buddies(cfs_rq, se);
> >
> > - if (se != cfs_rq->curr)
> > + if (se != cfs_rq->curr) {
> > __dequeue_entity(cfs_rq, se);
> > + __dequeue_latency(cfs_rq, se);
> > + }
> > se->on_rq = 0;
> > account_entity_dequeue(cfs_rq, se);
> >
> > @@ -4941,6 +5023,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > */
> > update_stats_wait_end_fair(cfs_rq, se);
> > __dequeue_entity(cfs_rq, se);
> > + __dequeue_latency(cfs_rq, se);
> > update_load_avg(cfs_rq, se, UPDATE_TG);
> > }
> >
> > @@ -4979,7 +5062,7 @@ 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 *latency, *se;
> >
> > /*
> > * If curr is set we have to see if its left of the leftmost entity
> > @@ -5021,6 +5104,12 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > se = cfs_rq->last;
> > }
> >
> > + /* Check for latency sensitive entity waiting for running */
> > + latency = __pick_first_latency(cfs_rq);
> > + if (latency && (latency != se) &&
> > + wakeup_preempt_entity(latency, se) < 1)
> > + se = latency;
> > +
> > return se;
> > }
> >
> > @@ -5044,6 +5133,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
> > update_stats_wait_start_fair(cfs_rq, prev);
> > /* Put 'current' back into the tree. */
> > __enqueue_entity(cfs_rq, prev);
> > + __enqueue_latency(cfs_rq, prev, 0);
> > /* in !on_rq case, update occurred at dequeue */
> > update_load_avg(cfs_rq, prev, 0);
> > }
> > @@ -12222,6 +12312,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
> > void init_cfs_rq(struct cfs_rq *cfs_rq)
> > {
> > cfs_rq->tasks_timeline = RB_ROOT_CACHED;
> > + cfs_rq->latency_timeline = RB_ROOT_CACHED;
> > u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20)));
> > #ifdef CONFIG_SMP
> > raw_spin_lock_init(&cfs_rq->removed.lock);
> > @@ -12378,6 +12469,7 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
> > se->my_q = cfs_rq;
> >
> > se->latency_offset = calc_latency_offset(tg->latency_prio);
> > + RB_CLEAR_NODE(&se->latency_node);
> >
> > /* guarantee group entities always have weight */
> > update_load_set(&se->load, NICE_0_LOAD);
> > @@ -12529,8 +12621,19 @@ int sched_group_set_latency(struct task_group *tg, int prio)
> >
> > for_each_possible_cpu(i) {
> > struct sched_entity *se = tg->se[i];
> > + struct rq *rq = cpu_rq(i);
> > + struct rq_flags rf;
> > + bool queued;
> > +
> > + rq_lock_irqsave(rq, &rf);
> >
> > + queued = __dequeue_latency(se->cfs_rq, se);
> > WRITE_ONCE(se->latency_offset, latency_offset);
> > + if (queued)
> > + __enqueue_latency(se->cfs_rq, se, ENQUEUE_WAKEUP);
> > +
> > +
> > + rq_unlock_irqrestore(rq, &rf);
> > }
> >
> > mutex_unlock(&shares_mutex);
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 9a2e71231083..21dd309e98a9 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -570,6 +570,7 @@ struct cfs_rq {
> > #endif
> >
> > struct rb_root_cached tasks_timeline;
> > + struct rb_root_cached latency_timeline;
> >
> > /*
> > * 'curr' points to currently running entity on this cfs_rq.
>

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In reply to: shrikanth hegde: "Re: [PATCH v12 8/8] sched/fair: Add latency list"
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