On 3/2/23 1:20 PM, Vincent Guittot wrote:
> 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
hackbench still seems to regress in different latency nice values compared to
baseline of 6.2 in this case. up to 50% in some cases.
12 core powerpc system with SMT=8 i.e 96 CPU
running 2 CPU cgroups. No quota assigned.
1st cgroup is running stress-ng with 48 threads. Consuming 50% of CPU.
latency is not changed for this cgroup.
2nd cgroup is running hackbench. This cgroup is assigned the different latency
nice values of 0, -20 and 19.
Numbers are average of 10 runs in each case. Time is in seconds
type groups | v6.2 | v6.2 + V12 | v6.2 + V12 | v6.2 + V12
| | lat nice=0 | lat nice=-20| lat nice=+19
| | | |
Process 10 | 0.36 | 0.41 | 0.43 | 0.42
Process 20 | 0.62 | 0.76 | 0.75 | 0.75
Process 30 | 0.87 | 1.05 | 1.04 | 1.06
Process 40 | 1.13 | 1.34 | 1.33 | 1.33
Process 50 | 1.38 | 1.62 | 1.66 | 1.63
Process 60 | 1.64 | 1.91 | 1.97 | 1.90
thread 10 | 0.35 | 0.41 | 0.44 | 0.42
thread 20 | 0.64 | 0.78 | 0.77 | 0.79
Process(Pipe) 10 | 0.20 | 0.34 | 0.33 | 0.34
Process(Pipe) 20 | 0.32 | 0.52 | 0.53 | 0.52
Process(Pipe) 30 | 0.44 | 0.70 | 0.70 | 0.69
Process(Pipe) 40 | 0.56 | 0.88 | 0.89 | 0.88
Process(Pipe) 50 | 0.70 | 1.08 | 1.08 | 1.07
Process(Pipe) 60 | 0.83 | 1.27 | 1.27 | 1.26
thread(Pipe) 10 | 0.21 | 0.35 | 0.34 | 0.36
thread(Pipe) 10 | 0.35 | 0.55 | 0.58 | 0.55
>> 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.
