Re: [RFC PATCH 1/2] sched/fair: introduce new scheduler group type group_parked

Shrikanth Hegde
2024-12-05T18:04:52Z

On 12/4/24 16:51, Tobias Huschle wrote:
> A parked CPU is considered to be flagged as unsuitable to process
> workload at the moment, but might be become usable anytime. Depending on
> the necessity for additional computation power and/or available capacity
> of the underlying hardware.
>
> A scheduler group is considered to be parked if it only contains parked
> CPUs. A parked scheduler group is considered to be busier than another
> if it runs more tasks than the other parked scheduler group.
>
> Indicators whether a CPU should be parked depend on the underlying
> hardware and must be considered to be architecture dependent.
> Therefore the check whether a CPU is parked is architecture specific.
> For architectures not relying on this feature, the check is a NOP.
>
> This is more efficient and non-disruptive compared to CPU hotplug in
> environments where such changes can be necessary on a frequent basis.
>
> Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
> ---
> include/linux/sched/topology.h | 20 ++++++
> kernel/sched/core.c | 10 ++-
> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++-------
> 3 files changed, 127 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 4237daa5ac7a..cfe3c59bc329 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -270,6 +270,26 @@ unsigned long arch_scale_cpu_capacity(int cpu)
> }
> #endif
>
> +#ifndef arch_cpu_parked
> +/**
> + * arch_cpu_parked - Check if a given CPU is currently parked.
> + *
> + * A parked CPU cannot run any kind of workload since underlying
> + * physical CPU should not be used at the moment .
> + *
> + * @cpu: the CPU in question.
> + *
> + * By default assume CPU is not parked
> + *
> + * Return: Parked state of CPU
> + */
> +static __always_inline
> +unsigned long arch_cpu_parked(int cpu)

bool instead?

> +{
> + return false;
> +}
> +#endif
> +
> #ifndef arch_scale_hw_pressure
> static __always_inline
> unsigned long arch_scale_hw_pressure(int cpu)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1dee3f5ef940..8f9aeb97c396 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2437,7 +2437,7 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>
> /* Non kernel threads are not allowed during either online or offline. */
> if (!(p->flags & PF_KTHREAD))
> - return cpu_active(cpu);
> + return !arch_cpu_parked(cpu) && cpu_active(cpu);
>
> /* KTHREAD_IS_PER_CPU is always allowed. */
> if (kthread_is_per_cpu(p))
> @@ -2447,6 +2447,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
> if (cpu_dying(cpu))
> return false;
>
> + /* CPU should be avoided at the moment */
> + if (arch_cpu_parked(cpu))
> + return false;
> +
> /* But are allowed during online. */
> return cpu_online(cpu);
> }
> @@ -3924,6 +3928,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
> if (task_on_scx(p))
> return false;
>
> + /* The task should not be queued onto a parked CPU. */
> + if (arch_cpu_parked(cpu))
> + return false;
> +

When it comes here, likely cpu is not parked since wakeup path has those
checks.

> /*
> * Do not complicate things with the async wake_list while the CPU is
> * in hotplug state.
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4283c818bbd1..fa1c19d285de 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7415,6 +7415,9 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> {
> int target = nr_cpumask_bits;
>
> + if (arch_cpu_parked(target))
> + return prev_cpu;
> +
> if (sched_feat(WA_IDLE))
> target = wake_affine_idle(this_cpu, prev_cpu, sync);
>
> @@ -7454,6 +7457,9 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
> for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
> struct rq *rq = cpu_rq(i);
>
> + if (arch_cpu_parked(i))
> + continue;
> +
> if (!sched_core_cookie_match(rq, p))
> continue;
>
> @@ -7546,10 +7552,14 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
> return new_cpu;
> }
>
> +static inline bool is_idle_cpu_allowed(int cpu)
> +{
> + return !arch_cpu_parked(cpu) && (available_idle_cpu(cpu) || sched_idle_cpu(cpu));
> +}

How about adding below code, it could simplify the code quite a bit. no?
sched_idle_rq also might need the same check though.

+++ b/kernel/sched/syscalls.c
@@ -214,6 +214,9 @@ int idle_cpu(int cpu)
return 0;
#endif

+ if (arch_cpu_parked(cpu))
+ return 0;
+
return 1;
}

> +
> static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> {
> - if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
> - sched_cpu_cookie_match(cpu_rq(cpu), p))
> + if (is_idle_cpu_allowed(cpu) && sched_cpu_cookie_match(cpu_rq(cpu), p))
> return cpu;
>
> return -1;
> @@ -7657,7 +7667,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
> */
> if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> continue;
> - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> + if (is_idle_cpu_allowed(cpu))
> return cpu;
> }
>
> @@ -7779,7 +7789,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> for_each_cpu_wrap(cpu, cpus, target) {
> unsigned long cpu_cap = capacity_of(cpu);
>
> - if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> + if (!is_idle_cpu_allowed(cpu))
> continue;
>
> fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> @@ -7850,7 +7860,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> */
> lockdep_assert_irqs_disabled();
>
> - if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
> + if (is_idle_cpu_allowed(target) &&
> asym_fits_cpu(task_util, util_min, util_max, target))
> return target;
>
> @@ -7858,7 +7868,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> * If the previous CPU is cache affine and idle, don't be stupid:
> */
> if (prev != target && cpus_share_cache(prev, target) &&
> - (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> + is_idle_cpu_allowed(prev) &&
> asym_fits_cpu(task_util, util_min, util_max, prev)) {
>
> if (!static_branch_unlikely(&sched_cluster_active) ||
> @@ -7890,7 +7900,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> if (recent_used_cpu != prev &&
> recent_used_cpu != target &&
> cpus_share_cache(recent_used_cpu, target) &&
> - (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
> + is_idle_cpu_allowed(recent_used_cpu) &&
> cpumask_test_cpu(recent_used_cpu, p->cpus_ptr) &&
> asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
>
> @@ -9198,7 +9208,12 @@ enum group_type {
> * The CPU is overloaded and can't provide expected CPU cycles to all
> * tasks.
> */
> - group_overloaded
> + group_overloaded,
> + /*
> + * The CPU should be avoided as it can't provide expected CPU cycles
> + * even for small amounts of workload.
> + */
> + group_parked
> };
>
> enum migration_type {
> @@ -9498,7 +9513,7 @@ static int detach_tasks(struct lb_env *env)
> * Source run queue has been emptied by another CPU, clear
> * LBF_ALL_PINNED flag as we will not test any task.
> */
> - if (env->src_rq->nr_running <= 1) {
> + if (env->src_rq->nr_running <= 1 && !arch_cpu_parked(env->src_cpu)) {
> env->flags &= ~LBF_ALL_PINNED;
> return 0;
> }
> @@ -9511,7 +9526,7 @@ static int detach_tasks(struct lb_env *env)
> * We don't want to steal all, otherwise we may be treated likewise,
> * which could at worst lead to a livelock crash.
> */
> - if (env->idle && env->src_rq->nr_running <= 1)
> + if (env->idle && env->src_rq->nr_running <= 1 && !arch_cpu_parked(env->src_cpu))
> break;
>
> env->loop++;
> @@ -9870,6 +9885,8 @@ struct sg_lb_stats {
> unsigned long group_runnable; /* Total runnable time over the CPUs of the group */
> unsigned int sum_nr_running; /* Nr of all tasks running in the group */
> unsigned int sum_h_nr_running; /* Nr of CFS tasks running in the group */
> + unsigned int sum_nr_parked;
> + unsigned int parked_cpus;

Can you please explain why you need two of these? Is it to identify the
group with most parked cpus? maybe comments is needed.

> unsigned int idle_cpus; /* Nr of idle CPUs in the group */
> unsigned int group_weight;
> enum group_type group_type;
> @@ -10127,6 +10144,9 @@ group_type group_classify(unsigned int imbalance_pct,
> struct sched_group *group,
> struct sg_lb_stats *sgs)
> {
> + if (sgs->parked_cpus)
> + return group_parked;
> +
> if (group_is_overloaded(imbalance_pct, sgs))
> return group_overloaded;
>
> @@ -10328,10 +10348,15 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> sgs->nr_numa_running += rq->nr_numa_running;
> sgs->nr_preferred_running += rq->nr_preferred_running;
> #endif
> +
> + if (rq->cfs.h_nr_running) {
> + sgs->parked_cpus += arch_cpu_parked(i);
> + sgs->sum_nr_parked += arch_cpu_parked(i) * rq->cfs.h_nr_running;
> + }
> /*
> * No need to call idle_cpu() if nr_running is not 0
> */
> - if (!nr_running && idle_cpu(i)) {
> + if (!nr_running && idle_cpu(i) && !arch_cpu_parked(i)) {
> sgs->idle_cpus++;
> /* Idle cpu can't have misfit task */
> continue;
> @@ -10355,7 +10380,14 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>
> sgs->group_capacity = group->sgc->capacity;
>
> - sgs->group_weight = group->group_weight;
> + sgs->group_weight = group->group_weight - sgs->parked_cpus;
> +
> + /*
> + * Only a subset of the group is parked, so the group itself has the
> + * capability to potentially pull tasks
> + */
> + if (sgs->parked_cpus < group->group_weight)
> + sgs->parked_cpus = 0;

Say you had a group with 4 cpus and 2 were parked CPUs. Now the
group_weight will be 2 and it will be marked as parked. whereas if 1 CPU
is parked group will not be marked as parked. That seems wrong.

instead mark it as parked and use the parked_cpus number to compare no?

>
> /* Check if dst CPU is idle and preferred to this group */
> if (!local_group && env->idle && sgs->sum_h_nr_running &&
> @@ -10422,6 +10454,8 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> */
>
> switch (sgs->group_type) {
> + case group_parked:
> + return sgs->sum_nr_parked > busiest->sum_nr_parked;
> case group_overloaded:
> /* Select the overloaded group with highest avg_load. */
> return sgs->avg_load > busiest->avg_load;
> @@ -10633,6 +10667,9 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
> nr_running = rq->nr_running - local;
> sgs->sum_nr_running += nr_running;
>
> + sgs->parked_cpus += arch_cpu_parked(i);
> + sgs->sum_nr_parked += arch_cpu_parked(i) * rq->cfs.h_nr_running;
> +
> /*
> * No need to call idle_cpu_without() if nr_running is not 0
> */
> @@ -10649,7 +10686,14 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
>
> sgs->group_capacity = group->sgc->capacity;
>
> - sgs->group_weight = group->group_weight;
> + sgs->group_weight = group->group_weight - sgs->parked_cpus;
> +
> + /*
> + * Only a subset of the group is parked, so the group itself has the
> + * capability to potentially pull tasks
> + */
> + if (sgs->parked_cpus < group->group_weight)
> + sgs->parked_cpus = 0;

same comment as above.

>
> sgs->group_type = group_classify(sd->imbalance_pct, group, sgs);
>
> @@ -10680,6 +10724,8 @@ static bool update_pick_idlest(struct sched_group *idlest,
> */
>
> switch (sgs->group_type) {
> + case group_parked:
> + return false;

Why not use the parked_cpus to compare?

> case group_overloaded:
> case group_fully_busy:
> /* Select the group with lowest avg_load. */
> @@ -10730,7 +10776,7 @@ sched_balance_find_dst_group(struct sched_domain *sd, struct task_struct *p, int
> unsigned long imbalance;
> struct sg_lb_stats idlest_sgs = {
> .avg_load = UINT_MAX,
> - .group_type = group_overloaded,
> + .group_type = group_parked,
> };
>
> do {
> @@ -10788,6 +10834,8 @@ sched_balance_find_dst_group(struct sched_domain *sd, struct task_struct *p, int
> return idlest;
>
> switch (local_sgs.group_type) {
> + case group_parked:
> + return idlest;
> case group_overloaded:
> case group_fully_busy:
>
> @@ -11039,6 +11087,12 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> local = &sds->local_stat;
> busiest = &sds->busiest_stat;
>
> + if (busiest->group_type == group_parked) {
> + env->migration_type = migrate_task;
> + env->imbalance = busiest->sum_nr_parked;
> + return;
> + }
> +
> if (busiest->group_type == group_misfit_task) {
> if (env->sd->flags & SD_ASYM_CPUCAPACITY) {
> /* Set imbalance to allow misfit tasks to be balanced. */
> @@ -11207,13 +11261,14 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> /*
> * Decision matrix according to the local and busiest group type:
> *
> - * busiest \ local has_spare fully_busy misfit asym imbalanced overloaded
> - * has_spare nr_idle balanced N/A N/A balanced balanced
> - * fully_busy nr_idle nr_idle N/A N/A balanced balanced
> - * misfit_task force N/A N/A N/A N/A N/A
> - * asym_packing force force N/A N/A force force
> - * imbalanced force force N/A N/A force force
> - * overloaded force force N/A N/A force avg_load
> + * busiest \ local has_spare fully_busy misfit asym imbalanced overloaded parked
> + * has_spare nr_idle balanced N/A N/A balanced balanced balanced
> + * fully_busy nr_idle nr_idle N/A N/A balanced balanced balanced
> + * misfit_task force N/A N/A N/A N/A N/A N/A
> + * asym_packing force force N/A N/A force force balanced
> + * imbalanced force force N/A N/A force force balanced
> + * overloaded force force N/A N/A force avg_load balanced
> + * parked force force N/A N/A force force nr_tasks

If i see the code below, if local is parked, it always goes to balanced.
how it is nr_tasks? am i reading this table wrong?

> *
> * N/A : Not Applicable because already filtered while updating
> * statistics.
> @@ -11222,6 +11277,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> * avg_load : Only if imbalance is significant enough.
> * nr_idle : dst_cpu is not busy and the number of idle CPUs is quite
> * different in groups.
> + * nr_task : balancing can go either way depending on the number of running tasks
> + * per group
> */
>
> /**
> @@ -11252,6 +11309,13 @@ static struct sched_group *sched_balance_find_src_group(struct lb_env *env)
> goto out_balanced;
>
> busiest = &sds.busiest_stat;
> + local = &sds.local_stat;
> +
> + if (local->group_type == group_parked)
> + goto out_balanced;
> +
> + if (busiest->group_type == group_parked)
> + goto force_balance;
>
> /* Misfit tasks should be dealt with regardless of the avg load */
> if (busiest->group_type == group_misfit_task)
> @@ -11273,7 +11337,6 @@ static struct sched_group *sched_balance_find_src_group(struct lb_env *env)
> if (busiest->group_type == group_imbalanced)
> goto force_balance;
>
> - local = &sds.local_stat;
> /*
> * If the local group is busier than the selected busiest group
> * don't try and pull any tasks.
> @@ -11386,6 +11449,8 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
> enum fbq_type rt;
>
> rq = cpu_rq(i);
> + if (arch_cpu_parked(i) && rq->cfs.h_nr_running)
> + return rq;
> rt = fbq_classify_rq(rq);
>
> /*
> @@ -11556,6 +11621,9 @@ static int need_active_balance(struct lb_env *env)
> {
> struct sched_domain *sd = env->sd;
>
> + if (arch_cpu_parked(env->src_cpu) && !idle_cpu(env->src_cpu))
> + return 1;
> +
> if (asym_active_balance(env))
> return 1;
>
> @@ -11589,6 +11657,9 @@ static int should_we_balance(struct lb_env *env)
> struct sched_group *sg = env->sd->groups;
> int cpu, idle_smt = -1;
>
> + if (arch_cpu_parked(env->dst_cpu))
> + return 0;
> +
> /*
> * Ensure the balancing environment is consistent; can happen
> * when the softirq triggers 'during' hotplug.
> @@ -11612,7 +11683,7 @@ static int should_we_balance(struct lb_env *env)
> cpumask_copy(swb_cpus, group_balance_mask(sg));
> /* Try to find first idle CPU */
> for_each_cpu_and(cpu, swb_cpus, env->cpus) {
> - if (!idle_cpu(cpu))
> + if (!idle_cpu(cpu) || arch_cpu_parked(cpu))
> continue;
>
> /*
> @@ -11707,7 +11778,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
> ld_moved = 0;
> /* Clear this flag as soon as we find a pullable task */
> env.flags |= LBF_ALL_PINNED;
> - if (busiest->nr_running > 1) {
> + if (busiest->nr_running > 1 || arch_cpu_parked(busiest->cpu)) {
> /*
> * Attempt to move tasks. If sched_balance_find_src_group has found
> * an imbalance but busiest->nr_running <= 1, the group is
> @@ -12721,6 +12792,9 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
>
> update_misfit_status(NULL, this_rq);
>
> + if (arch_cpu_parked(this_cpu))
> + return 0;
> +
> /*
> * There is a task waiting to run. No need to search for one.
> * Return 0; the task will be enqueued when switching to idle.