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

From: Vincent Guittot
Date: Thu Mar 02 2023 - 08:18:17 EST

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On Thu, 2 Mar 2023 at 12:00, Shrikanth Hegde <sshegde@xxxxxxxxxxxxxxxxxx> wrote:
>
>
> 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.

According to your other emails, you are using the cgroup interface and
not the task's one. Do I get it right ?

I haven't run test such tests in a cgroup but at least the test with
latency_nice == 0 should not make any noticeable difference. Does this
include the re-arrange patch that you have proposed previously ?

Also, the tests that you did on v6, gave better result.
https://lore.kernel.org/lkml/34112324-de67-55eb-92bc-181a98c4311c@xxxxxxxxxxxxxxxxxx/

Are you running same tests or you changed something in the mean time ?

>
> 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.
>

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