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    SubjectRe: [PATCH v2 03/12] sched/core: uclamp: add CPU's clamp groups accounting
    Hi Patrick,

    On Mon, Jul 16, 2018 at 1:28 AM, Patrick Bellasi
    <patrick.bellasi@arm.com> wrote:
    > Utilization clamping allows to clamp the utilization of a CPU within a
    > [util_min, util_max] range. This range depends on the set of currently
    > RUNNABLE tasks on a CPU, where each task references two "clamp groups"
    > defining the util_min and the util_max clamp values to be considered for
    > that task. The clamp value mapped by a clamp group applies to a CPU only
    > when there is at least one task RUNNABLE referencing that clamp group.
    >
    > When tasks are enqueued/dequeued on/from a CPU, the set of clamp groups
    > active on that CPU can change. Since each clamp group enforces a
    > different utilization clamp value, once the set of these groups changes
    > it can be required to re-compute what is the new "aggregated" clamp
    > value to apply on that CPU.
    >
    > Clamp values are always MAX aggregated for both util_min and util_max.
    > This is to ensure that no tasks can affect the performance of other
    > co-scheduled tasks which are either more boosted (i.e. with higher
    > util_min clamp) or less capped (i.e. with higher util_max clamp).
    >
    > Here we introduce the required support to properly reference count clamp
    > groups at each task enqueue/dequeue time.
    >
    > Tasks have a:
    > task_struct::uclamp::group_id[clamp_idx]
    > indexing, for each clamp index (i.e. util_{min,max}), the clamp group in
    > which they should refcount at enqueue time.
    >
    > CPUs rq have a:
    > rq::uclamp::group[clamp_idx][group_idx].tasks
    > which is used to reference count how many tasks are currently RUNNABLE on
    > that CPU for each clamp group of each clamp index..
    >
    > The clamp value of each clamp group is tracked by
    > rq::uclamp::group[][].value, thus making rq::uclamp::group[][] an
    > unordered array of clamp values. However, the MAX aggregation of the
    > currently active clamp groups is implemented to minimize the number of
    > times we need to scan the complete (unordered) clamp group array to
    > figure out the new max value. This operation indeed happens only when we
    > dequeue last task of the clamp group corresponding to the current max
    > clamp, and thus the CPU is either entering IDLE or going to schedule a
    > less boosted or more clamped task.
    > Moreover, the expected number of different clamp values, which can be
    > configured at build time, is usually so small that a more advanced
    > ordering algorithm is not needed. In real use-cases we expect less then
    > 10 different values.
    >
    > Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
    > Cc: Ingo Molnar <mingo@redhat.com>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Paul Turner <pjt@google.com>
    > Cc: Todd Kjos <tkjos@google.com>
    > Cc: Joel Fernandes <joelaf@google.com>
    > Cc: Juri Lelli <juri.lelli@redhat.com>
    > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
    > Cc: Morten Rasmussen <morten.rasmussen@arm.com>
    > Cc: linux-kernel@vger.kernel.org
    > Cc: linux-pm@vger.kernel.org
    > ---
    > kernel/sched/core.c | 188 +++++++++++++++++++++++++++++++++++++++++++
    > kernel/sched/fair.c | 4 +
    > kernel/sched/rt.c | 4 +
    > kernel/sched/sched.h | 71 ++++++++++++++++
    > 4 files changed, 267 insertions(+)
    >
    > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    > index 50e749067df5..d1969931fea6 100644
    > --- a/kernel/sched/core.c
    > +++ b/kernel/sched/core.c
    > @@ -848,9 +848,19 @@ static inline void uclamp_group_init(int clamp_id, int group_id,
    > unsigned int clamp_value)
    > {
    > struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
    > + struct uclamp_cpu *uc_cpu;
    > + int cpu;
    >
    > + /* Set clamp group map */
    > uc_map[group_id].value = clamp_value;
    > uc_map[group_id].se_count = 0;
    > +
    > + /* Set clamp groups on all CPUs */
    > + for_each_possible_cpu(cpu) {
    > + uc_cpu = &cpu_rq(cpu)->uclamp;
    > + uc_cpu->group[clamp_id][group_id].value = clamp_value;
    > + uc_cpu->group[clamp_id][group_id].tasks = 0;
    > + }
    > }
    >
    > /**
    > @@ -906,6 +916,172 @@ uclamp_group_find(int clamp_id, unsigned int clamp_value)
    > return group_id;
    > }
    >
    > +/**
    > + * uclamp_cpu_update: updates the utilization clamp of a CPU
    > + * @cpu: the CPU which utilization clamp has to be updated
    > + * @clamp_id: the clamp index to update
    > + *
    > + * When tasks are enqueued/dequeued on/from a CPU, the set of currently active
    > + * clamp groups is subject to change. Since each clamp group enforces a
    > + * different utilization clamp value, once the set of these groups changes it
    > + * can be required to re-compute what is the new clamp value to apply for that
    > + * CPU.
    > + *
    > + * For the specified clamp index, this method computes the new CPU utilization
    > + * clamp to use until the next change on the set of RUNNABLE tasks on that CPU.
    > + */
    > +static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
    > +{
    > + struct uclamp_group *uc_grp = &rq->uclamp.group[clamp_id][0];
    > + int max_value = UCLAMP_NONE;
    > + unsigned int group_id;
    > +
    > + for (group_id = 0; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; ++group_id) {
    > + /* Ignore inactive clamp groups, i.e. no RUNNABLE tasks */
    > + if (!uclamp_group_active(uc_grp, group_id))
    > + continue;
    > +
    > + /* Both min and max clamp are MAX aggregated */
    > + max_value = max(max_value, uc_grp[group_id].value);
    > +
    > + /* Stop if we reach the max possible clamp */
    > + if (max_value >= SCHED_CAPACITY_SCALE)
    > + break;
    > + }
    > + rq->uclamp.value[clamp_id] = max_value;
    > +}
    > +
    > +/**
    > + * uclamp_cpu_get_id(): increase reference count for a clamp group on a CPU
    > + * @p: the task being enqueued on a CPU
    > + * @rq: the CPU's rq where the clamp group has to be reference counted
    > + * @clamp_id: the utilization clamp (e.g. min or max utilization) to reference
    > + *
    > + * Once a task is enqueued on a CPU's RQ, the clamp group currently defined by
    > + * the task's uclamp.group_id is reference counted on that CPU.
    > + */
    > +static inline void uclamp_cpu_get_id(struct task_struct *p,
    > + struct rq *rq, int clamp_id)
    > +{
    > + struct uclamp_group *uc_grp;
    > + struct uclamp_cpu *uc_cpu;
    > + int clamp_value;
    > + int group_id;
    > +
    > + /* No task specific clamp values: nothing to do */
    > + group_id = p->uclamp[clamp_id].group_id;
    > + if (group_id == UCLAMP_NONE)
    > + return;
    > +
    > + /* Reference count the task into its current group_id */
    > + uc_grp = &rq->uclamp.group[clamp_id][0];
    > + uc_grp[group_id].tasks += 1;
    > +
    > + /*
    > + * If this is the new max utilization clamp value, then we can update
    > + * straight away the CPU clamp value. Otherwise, the current CPU clamp
    > + * value is still valid and we are done.
    > + */
    > + uc_cpu = &rq->uclamp;
    > + clamp_value = p->uclamp[clamp_id].value;
    > + if (uc_cpu->value[clamp_id] < clamp_value)
    > + uc_cpu->value[clamp_id] = clamp_value;
    > +}
    > +
    > +/**
    > + * uclamp_cpu_put_id(): decrease reference count for a clamp group on a CPU
    > + * @p: the task being dequeued from a CPU
    > + * @cpu: the CPU from where the clamp group has to be released
    > + * @clamp_id: the utilization clamp (e.g. min or max utilization) to release
    > + *
    > + * When a task is dequeued from a CPU's RQ, the CPU's clamp group reference
    > + * counted by the task is decreased.
    > + * If this was the last task defining the current max clamp group, then the
    > + * CPU clamping is updated to find the new max for the specified clamp
    > + * index.
    > + */
    > +static inline void uclamp_cpu_put_id(struct task_struct *p,
    > + struct rq *rq, int clamp_id)
    > +{
    > + struct uclamp_group *uc_grp;
    > + struct uclamp_cpu *uc_cpu;
    > + unsigned int clamp_value;
    > + int group_id;
    > +
    > + /* No task specific clamp values: nothing to do */
    > + group_id = p->uclamp[clamp_id].group_id;
    > + if (group_id == UCLAMP_NONE)
    > + return;
    > +
    > + /* Decrement the task's reference counted group index */
    > + uc_grp = &rq->uclamp.group[clamp_id][0];
    > + uc_grp[group_id].tasks -= 1;
    > +
    > + /* If this is not the last task, no updates are required */
    > + if (uc_grp[group_id].tasks > 0)
    > + return;
    > +
    > + /*
    > + * Update the CPU only if this was the last task of the group
    > + * defining the current clamp value.
    > + */
    > + uc_cpu = &rq->uclamp;
    > + clamp_value = uc_grp[group_id].value;
    > + if (clamp_value >= uc_cpu->value[clamp_id])
    > + uclamp_cpu_update(rq, clamp_id);
    > +}
    > +
    > +/**
    > + * uclamp_cpu_get(): increase CPU's clamp group refcount
    > + * @rq: the CPU's rq where the clamp group has to be refcounted
    > + * @p: the task being enqueued
    > + *
    > + * Once a task is enqueued on a CPU's rq, all the clamp groups currently
    > + * enforced on a task are reference counted on that rq.
    > + * Not all scheduling classes have utilization clamping support, their tasks
    > + * will be silently ignored.
    > + *
    > + * This method updates the utilization clamp constraints considering the
    > + * requirements for the specified task. Thus, this update must be done before
    > + * calling into the scheduling classes, which will eventually update schedutil
    > + * considering the new task requirements.
    > + */
    > +static inline void uclamp_cpu_get(struct rq *rq, struct task_struct *p)
    > +{
    > + int clamp_id;
    > +
    > + if (unlikely(!p->sched_class->uclamp_enabled))
    > + return;
    > +
    > + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id)
    > + uclamp_cpu_get_id(p, rq, clamp_id);
    > +}
    > +
    > +/**
    > + * uclamp_cpu_put(): decrease CPU's clamp group refcount
    > + * @cpu: the CPU's rq where the clamp group refcount has to be decreased
    > + * @p: the task being dequeued
    > + *
    > + * When a task is dequeued from a CPU's rq, all the clamp groups the task has
    > + * been reference counted at task's enqueue time have to be decreased for that
    > + * CPU.
    > + *
    > + * This method updates the utilization clamp constraints considering the
    > + * requirements for the specified task. Thus, this update must be done before
    > + * calling into the scheduling classes, which will eventually update schedutil
    > + * considering the new task requirements.
    > + */
    > +static inline void uclamp_cpu_put(struct rq *rq, struct task_struct *p)
    > +{
    > + int clamp_id;
    > +
    > + if (unlikely(!p->sched_class->uclamp_enabled))
    > + return;
    > +
    > + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id)
    > + uclamp_cpu_put_id(p, rq, clamp_id);
    > +}
    > +
    > /**
    > * uclamp_group_put: decrease the reference count for a clamp group
    > * @clamp_id: the clamp index which was affected by a task group
    > @@ -1021,9 +1197,17 @@ static inline int __setscheduler_uclamp(struct task_struct *p,
    > static void __init init_uclamp(void)
    > {
    > int clamp_id;
    > + int cpu;
    >
    > mutex_init(&uclamp_mutex);
    >
    > + /* Init CPU's clamp groups */
    > + for_each_possible_cpu(cpu) {
    > + struct uclamp_cpu *uc_cpu = &cpu_rq(cpu)->uclamp;
    > +
    > + memset(uc_cpu, UCLAMP_NONE, sizeof(struct uclamp_cpu));
    > + }
    > +
    > /* Init SE's clamp map */
    > for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
    > struct uclamp_map *uc_map = &uclamp_maps[clamp_id][0];
    > @@ -1037,6 +1221,8 @@ static void __init init_uclamp(void)
    > }
    >
    > #else /* CONFIG_UCLAMP_TASK */
    > +static inline void uclamp_cpu_get(struct rq *rq, struct task_struct *p) { }
    > +static inline void uclamp_cpu_put(struct rq *rq, struct task_struct *p) { }
    > static inline int __setscheduler_uclamp(struct task_struct *p,
    > const struct sched_attr *attr)
    > {
    > @@ -1053,6 +1239,7 @@ static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
    > if (!(flags & ENQUEUE_RESTORE))
    > sched_info_queued(rq, p);
    >
    > + uclamp_cpu_get(rq, p);
    > p->sched_class->enqueue_task(rq, p, flags);
    > }
    >
    > @@ -1064,6 +1251,7 @@ static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
    > if (!(flags & DEQUEUE_SAVE))
    > sched_info_dequeued(rq, p);
    >
    > + uclamp_cpu_put(rq, p);
    > p->sched_class->dequeue_task(rq, p, flags);
    > }
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 2f0a0be4d344..fd857440276c 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -10433,6 +10433,10 @@ const struct sched_class fair_sched_class = {
    > #ifdef CONFIG_FAIR_GROUP_SCHED
    > .task_change_group = task_change_group_fair,
    > #endif
    > +
    > +#ifdef CONFIG_UCLAMP_TASK
    > + .uclamp_enabled = 1,
    > +#endif
    > };
    >
    > #ifdef CONFIG_SCHED_DEBUG
    > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    > index 572567078b60..056a7e1bd529 100644
    > --- a/kernel/sched/rt.c
    > +++ b/kernel/sched/rt.c
    > @@ -2391,6 +2391,10 @@ const struct sched_class rt_sched_class = {
    > .switched_to = switched_to_rt,
    >
    > .update_curr = update_curr_rt,
    > +
    > +#ifdef CONFIG_UCLAMP_TASK
    > + .uclamp_enabled = 1,
    > +#endif
    > };
    >
    > #ifdef CONFIG_RT_GROUP_SCHED
    > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    > index c7742dcc136c..65bf9ebacd83 100644
    > --- a/kernel/sched/sched.h
    > +++ b/kernel/sched/sched.h
    > @@ -744,6 +744,50 @@ extern void rto_push_irq_work_func(struct irq_work *work);
    > #endif
    > #endif /* CONFIG_SMP */
    >
    > +#ifdef CONFIG_UCLAMP_TASK
    > +/**
    > + * struct uclamp_group - Utilization clamp Group
    > + * @value: utilization clamp value for tasks on this clamp group
    > + * @tasks: number of RUNNABLE tasks on this clamp group
    > + *
    > + * Keep track of how many tasks are RUNNABLE for a given utilization
    > + * clamp value.
    > + */
    > +struct uclamp_group {
    > + int value;
    > + int tasks;
    > +};
    > +
    > +/**
    > + * struct uclamp_cpu - CPU's utilization clamp
    > + * @value: currently active clamp values for a CPU
    > + * @group: utilization clamp groups affecting a CPU
    > + *
    > + * Keep track of RUNNABLE tasks on a CPUs to aggregate their clamp values.
    > + * A clamp value is affecting a CPU where there is at least one task RUNNABLE
    > + * (or actually running) with that value.
    > + *
    > + * We have up to UCLAMP_CNT possible different clamp value, which are
    > + * currently only two: minmum utilization and maximum utilization.
    > + *
    > + * All utilization clamping values are MAX aggregated, since:
    > + * - for util_min: we want to run the CPU at least at the max of the minimum
    > + * utilization required by its currently RUNNABLE tasks.
    > + * - for util_max: we want to allow the CPU to run up to the max of the
    > + * maximum utilization allowed by its currently RUNNABLE tasks.
    > + *
    > + * Since on each system we expect only a limited number of different
    > + * utilization clamp values (CONFIG_UCLAMP_GROUPS_COUNT), we use a simple
    > + * array to track the metrics required to compute all the per-CPU utilization
    > + * clamp values. The additional slot is used to track the default clamp
    > + * values, i.e. no min/max clamping at all.
    > + */
    > +struct uclamp_cpu {
    > + int value[UCLAMP_CNT];
    > + struct uclamp_group group[UCLAMP_CNT][CONFIG_UCLAMP_GROUPS_COUNT + 1];
    > +};
    > +#endif /* CONFIG_UCLAMP_TASK */
    > +
    > /*
    > * This is the main, per-CPU runqueue data structure.
    > *
    > @@ -781,6 +825,11 @@ struct rq {
    > unsigned long nr_load_updates;
    > u64 nr_switches;
    >
    > +#ifdef CONFIG_UCLAMP_TASK
    > + /* Utilization clamp values based on CPU's RUNNABLE tasks */
    > + struct uclamp_cpu uclamp ____cacheline_aligned;
    > +#endif
    > +
    > struct cfs_rq cfs;
    > struct rt_rq rt;
    > struct dl_rq dl;
    > @@ -1535,6 +1584,10 @@ struct sched_class {
    > #ifdef CONFIG_FAIR_GROUP_SCHED
    > void (*task_change_group)(struct task_struct *p, int type);
    > #endif
    > +
    > +#ifdef CONFIG_UCLAMP_TASK
    > + int uclamp_enabled;
    > +#endif
    > };
    >
    > static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
    > @@ -2130,6 +2183,24 @@ static inline u64 irq_time_read(int cpu)
    > }
    > #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
    >
    > +#ifdef CONFIG_UCLAMP_TASK
    > +/**
    > + * uclamp_group_active: check if a clamp group is active on a CPU
    > + * @uc_grp: the clamp groups for a CPU
    > + * @group_id: the clamp group to check
    > + *
    > + * A clamp group affects a CPU if it as at least one RUNNABLE task.

    typo: "has at least"

    > + *
    > + * Return: true if the specified CPU has at least one RUNNABLE task
    > + * for the specified clamp group.
    > + */
    > +static inline bool uclamp_group_active(struct uclamp_group *uc_grp,
    > + int group_id)
    > +{
    > + return uc_grp[group_id].tasks > 0;
    > +}
    > +#endif /* CONFIG_UCLAMP_TASK */
    > +
    > #ifdef CONFIG_CPU_FREQ
    > DECLARE_PER_CPU(struct update_util_data *, cpufreq_update_util_data);
    >
    > --
    > 2.17.1
    >

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