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.
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.
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.
