Re: [RFC PATCH 0/2] sched/fair: introduce new scheduler group type group_parked

Tobias Huschle
2024-12-16T16:13:28Z

On 10/12/2024 21:24, Shrikanth Hegde wrote:
>
>
> On 12/9/24 13:35, Tobias Huschle wrote:
>>
> [...]
>>> So I gave it a try with using a debugfs based hint to say which CPUs
>>> are parked.
>>> It is a hack to try it out. patch is below so one could try something
>>> similar is their archs
>>> and see if it help if they have a use case.
>>>
>>> Notes:
>>> 1. Arch shouldn't set cpu_parked for all CPUs at boot. It causes panic.
>>> 2. Workload gets unpacked to all CPUs when changing from 40 CPUs to
>>> 80 CPUs, but
>>>     doesn't get packed when changing the from 80 to 40 CPUs.
>>
>> With stress-ng -l 100 this can happen, I tested with stress-ng -l 50
>> and that worked well in all cases. As mentioned above, the -l 100 case
>> would need changes to handle the no-hz scenario. I have a patch for
>> that which works, but it is a bit hacky.
>> If this also happens with non-100% stressors on your end, something
>> needs ot be fixed code-wise.
>>
>
> It was happening with 100% stress-ng case. I was wondering since i dont
> have no-hz full enabled.
> I found out the reason why and one way to do is to trigger active load
> balance if there are any parked cpus
> in the group. That probably needs a IS_ENABLED check not to hurt the
> regular case.
>
> Also, I gave a try to include arch_cpu_parked in idle_cpu and friends.
> It seems to working for me.
> I will attach the code below. It simplifies code quite a bit.

I agree, that this makes things cleaner. One concern might be that this
interferes with disabling the tick on idle CPUs. Will need to check.

>
> Also, I am thinking to rely on active balance codepath more than the
> regular pull model.
> so this would be akin to asym packing codepaths. The below code does
> that too.>
> Feel free to take the bits as necessary if it works.

I appreciate your suggestions. Experimenting with these changes worked
fine for me so far. I'll try to remove as many checks as possible and
hopefully be able to provide a v2 soon.

Things that I observed: With your changes incorporated and some things
that I added, it seems to work pretty fine, even with stress-ng using
100% stressors. But, it occasionally takes a while to reclaim unparked
CPUs. With any percentage below 100, the changes are almost instantly.

I also see some issues where the load balancer is very hesitant to
reclaim unparked CPUs which are part of another scheduling domain.

>
> ---
>  include/linux/sched/topology.h | 20 ++++++++++
>  kernel/sched/core.c            |  6 ++-
>  kernel/sched/fair.c            | 72 ++++++++++++++++++++++++++++++++--
>  kernel/sched/syscalls.c        |  3 ++
>  4 files changed, 97 insertions(+), 4 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)
> +{
> +    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 5fbec67d48b2..78ca95aad66b 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);
>  }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d5127d9beaea..a6216f63b756 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6898,6 +6898,9 @@ static int sched_idle_rq(struct rq *rq)
>  #ifdef CONFIG_SMP
>  static int sched_idle_cpu(int cpu)
>  {
> +    if (arch_cpu_parked(cpu))
> +        return 0;
> +
>      return sched_idle_rq(cpu_rq(cpu));
>  }
>  #endif
> @@ -7415,6 +7418,9 @@ 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);
>
> @@ -9198,7 +9204,13 @@ 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 {
> @@ -9880,6 +9892,9 @@ struct sg_lb_stats {
>      unsigned int nr_numa_running;
>      unsigned int nr_preferred_running;
>  #endif
> +    unsigned int sum_nr_parked;
> +    unsigned int parked_cpus;
> +
>  };
>
>  /*
> @@ -10127,6 +10142,9 @@ group_type group_classify(unsigned int
> imbalance_pct,
>                struct sched_group *group,
>                struct sg_lb_stats *sgs)
>  {
> +    if (sgs->parked_cpus)
> +        return group_parked;
> +

This could be changed to:

if (sgs->sum_nr_parked)
return group_parked;

This makes sure that we don't care about parked CPUs which do not run
any tasks at the moment. Needs an additional check for the case that all
CPUs of a group are parked.

>      if (group_is_overloaded(imbalance_pct, sgs))
>          return group_overloaded;
>
> @@ -10328,6 +10346,11 @@ 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;
> +        }
> +

I'm still a bit torn on this, as in how to count the parked cpus and
make use of that count. I think they should be removed from the overall
weight of the group. Probably also from the capacity. Otherwise, the
calculations for overloaded groups are off. Will look at that.

>          /*
>           * No need to call idle_cpu() if nr_running is not 0
>           */
> @@ -10422,6 +10445,8 @@ 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 +10658,8 @@ 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
>           */
> @@ -10680,6 +10707,8 @@ static bool update_pick_idlest(struct
> sched_group *idlest,
>       */
>
>      switch (sgs->group_type) {
> +    case group_parked:
> +        return false;
>      case group_overloaded:
>      case group_fully_busy:
>          /* Select the group with lowest avg_load. */
> @@ -10730,7 +10759,7 @@ 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 +10817,8 @@ 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 +11070,12 @@ 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. */
> @@ -11252,6 +11289,13 @@ 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 +11317,6 @@ 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 +11429,9 @@ 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 +11602,9 @@ static int need_active_balance(struct lb_env *env)
>  {
>      struct sched_domain *sd = env->sd;
>
> +    if (arch_cpu_parked(env->src_cpu))
> +        return 1;
> +
>      if (asym_active_balance(env))
>          return 1;
>
> @@ -11588,6 +11637,20 @@ static int should_we_balance(struct lb_env *env)
>      struct cpumask *swb_cpus =
> this_cpu_cpumask_var_ptr(should_we_balance_tmpmask);
>      struct sched_group *sg = env->sd->groups;
>      int cpu, idle_smt = -1;
> +    int cpus_parked = 0;
> +
> +    if (arch_cpu_parked(env->dst_cpu))
> +        return 0;
> +
> +    for_each_cpu(cpu, sched_domain_span(env->sd)) {
> +        if (arch_cpu_parked(cpu)) {
> +            cpus_parked ++;
> +        }
> +    }
> +
> +    if (cpus_parked && !arch_cpu_parked(env->dst_cpu)) {
> +        return 1;
> +    }
>
>      /*
>       * Ensure the balancing environment is consistent; can happen
> @@ -12708,6 +12771,9 @@ 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.
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index ff0e5ab4e37c..d408d87da563 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -203,6 +203,9 @@ int idle_cpu(int cpu)
>  {
>      struct rq *rq = cpu_rq(cpu);
>
> +    if (arch_cpu_parked(cpu))
> +        return 0;
> +
>      if (rq->curr != rq->idle)
>          return 0;
>