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Re: [RFC 2/3] sched/fair: use util_est in LB

Started byPavan Kondeti <pkondeti@codeaurora.org>
First post2017-08-29 06:50 +0200
Last post2017-09-04 17:00 +0200
Articles 3 — 2 participants

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  Re: [RFC 2/3] sched/fair: use util_est in LB Pavan Kondeti <pkondeti@codeaurora.org> - 2017-08-29 06:50 +0200
    Re: [RFC 2/3] sched/fair: use util_est in LB Patrick Bellasi <patrick.bellasi@arm.com> - 2017-09-04 16:20 +0200
      Re: [RFC 2/3] sched/fair: use util_est in LB Pavan Kondeti <pkondeti@codeaurora.org> - 2017-09-04 17:00 +0200

#1722137 — Re: [RFC 2/3] sched/fair: use util_est in LB

FromPavan Kondeti <pkondeti@codeaurora.org>
Date2017-08-29 06:50 +0200
SubjectRe: [RFC 2/3] sched/fair: use util_est in LB
Message-ID<ujGDn-6rm-5@gated-at.bofh.it>
On Fri, Aug 25, 2017 at 3:50 PM, Patrick Bellasi
<patrick.bellasi@arm.com> wrote:
> When the scheduler looks at the CPU utlization, the current PELT value
> for a CPU is returned straight away. In certain scenarios this can have
> undesired side effects on task placement.
>

<snip>

> +/**
> + * cpu_util_est: estimated utilization for the specified CPU
> + * @cpu: the CPU to get the estimated utilization for
> + *
> + * The estimated utilization of a CPU is defined to be the maximum between its
> + * PELT's utilization and the sum of the estimated utilization of the tasks
> + * currently RUNNABLE on that CPU.
> + *
> + * This allows to properly represent the expected utilization of a CPU which
> + * has just got a big task running since a long sleep period. At the same time
> + * however it preserves the benefits of the "blocked load" in describing the
> + * potential for other tasks waking up on the same CPU.
> + *
> + * Return: the estimated utlization for the specified CPU
> + */
> +static inline unsigned long cpu_util_est(int cpu)
> +{
> +       struct sched_avg *sa = &cpu_rq(cpu)->cfs.avg;
> +       unsigned long util = cpu_util(cpu);
> +
> +       if (!sched_feat(UTIL_EST))
> +               return util;
> +
> +       return max(util, util_est(sa, UTIL_EST_LAST));
> +}
> +
>  static inline int task_util(struct task_struct *p)
>  {
>         return p->se.avg.util_avg;
> @@ -6007,11 +6033,19 @@ static int cpu_util_wake(int cpu, struct task_struct *p)
>
>         /* Task has no contribution or is new */
>         if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
> -               return cpu_util(cpu);
> +               return cpu_util_est(cpu);
>
>         capacity = capacity_orig_of(cpu);
>         util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
>
> +       /*
> +        * Estimated utilization tracks only tasks already enqueued, but still
> +        * sometimes can return a bigger value than PELT, for example when the
> +        * blocked load is negligible wrt the estimated utilization of the
> +        * already enqueued tasks.
> +        */
> +       util = max_t(long, util, cpu_util_est(cpu));
> +

We are supposed to discount the task's util from its CPU. But the
cpu_util_est() can potentially return cpu_util() which includes the
task's utilization.

Thanks,
Pavan

-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
Linux Foundation Collaborative Project

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#1726104

FromPatrick Bellasi <patrick.bellasi@arm.com>
Date2017-09-04 16:20 +0200
Message-ID<um0oh-6xl-1@gated-at.bofh.it>
In reply to#1722137
On 29-Aug 10:15, Pavan Kondeti wrote:
> On Fri, Aug 25, 2017 at 3:50 PM, Patrick Bellasi
> <patrick.bellasi@arm.com> wrote:
> > When the scheduler looks at the CPU utlization, the current PELT value
> > for a CPU is returned straight away. In certain scenarios this can have
> > undesired side effects on task placement.
> >
> 
> <snip>
> 
> > +/**
> > + * cpu_util_est: estimated utilization for the specified CPU
> > + * @cpu: the CPU to get the estimated utilization for
> > + *
> > + * The estimated utilization of a CPU is defined to be the maximum between its
> > + * PELT's utilization and the sum of the estimated utilization of the tasks
> > + * currently RUNNABLE on that CPU.
> > + *
> > + * This allows to properly represent the expected utilization of a CPU which
> > + * has just got a big task running since a long sleep period. At the same time
> > + * however it preserves the benefits of the "blocked load" in describing the
> > + * potential for other tasks waking up on the same CPU.
> > + *
> > + * Return: the estimated utlization for the specified CPU
> > + */
> > +static inline unsigned long cpu_util_est(int cpu)
> > +{
> > +       struct sched_avg *sa = &cpu_rq(cpu)->cfs.avg;
> > +       unsigned long util = cpu_util(cpu);
> > +
> > +       if (!sched_feat(UTIL_EST))
> > +               return util;
> > +
> > +       return max(util, util_est(sa, UTIL_EST_LAST));
> > +}
> > +
> >  static inline int task_util(struct task_struct *p)
> >  {
> >         return p->se.avg.util_avg;
> > @@ -6007,11 +6033,19 @@ static int cpu_util_wake(int cpu, struct task_struct *p)
> >
> >         /* Task has no contribution or is new */
> >         if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
> > -               return cpu_util(cpu);
> > +               return cpu_util_est(cpu);
> >
> >         capacity = capacity_orig_of(cpu);
> >         util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
> >
> > +       /*
> > +        * Estimated utilization tracks only tasks already enqueued, but still
> > +        * sometimes can return a bigger value than PELT, for example when the
> > +        * blocked load is negligible wrt the estimated utilization of the
> > +        * already enqueued tasks.
> > +        */
> > +       util = max_t(long, util, cpu_util_est(cpu));
> > +
> 
> We are supposed to discount the task's util from its CPU. But the
> cpu_util_est() can potentially return cpu_util() which includes the
> task's utilization.

You right, this instead should cover all the cases:

---8<---
 static int cpu_util_wake(int cpu, struct task_struct *p)
 {
-       unsigned long util, capacity;
+       unsigned long util_est = cpu_util_est(cpu);
+       unsigned long capacity;
 
        /* Task has no contribution or is new */
        if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
-               return cpu_util(cpu);
+               return util_est;
 
        capacity = capacity_orig_of(cpu);
-       util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
+       if (cpu_util(cpu) > util_est)
+               util = max_t(long, cpu_util(cpu) - task_util(p), 0);
+       else
+               util = util_est;
 
        return (util >= capacity) ? capacity : util;
 }
---8<---

Indeed:

- if *p is the only task sleeping on that CPU, then:
      (cpu_util == task_util) > (cpu_util_est == 0)
  and thus we return:
      (cpu_util - task_util) == 0

- if other tasks are SLEEPING on the same CPU, which however is IDLE, then:
      cpu_util > (cpu_util_est == 0)
  and thus we discount *p's blocked load by returning:
      (cpu_util - task_util) >= 0

- if other tasks are RUNNABLE on that CPU and
      (cpu_util_est > cpu_util)
  then we wanna use cpu_util_est since it returns a more restrictive
  estimation of the spare capacity on that CPU, by just considering
  the expected utilization of tasks already runnable on that CPU.

What do you think?

> Thanks,
> Pavan

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi

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#1726129

FromPavan Kondeti <pkondeti@codeaurora.org>
Date2017-09-04 17:00 +0200
Message-ID<um10Z-6Md-7@gated-at.bofh.it>
In reply to#1726104
On Mon, Sep 4, 2017 at 7:48 PM, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> On 29-Aug 10:15, Pavan Kondeti wrote:
>> On Fri, Aug 25, 2017 at 3:50 PM, Patrick Bellasi
>> <patrick.bellasi@arm.com> wrote:
>> > When the scheduler looks at the CPU utlization, the current PELT value
>> > for a CPU is returned straight away. In certain scenarios this can have
>> > undesired side effects on task placement.
>> >
>>
>> <snip>
>>
>> > +/**
>> > + * cpu_util_est: estimated utilization for the specified CPU
>> > + * @cpu: the CPU to get the estimated utilization for
>> > + *
>> > + * The estimated utilization of a CPU is defined to be the maximum between its
>> > + * PELT's utilization and the sum of the estimated utilization of the tasks
>> > + * currently RUNNABLE on that CPU.
>> > + *
>> > + * This allows to properly represent the expected utilization of a CPU which
>> > + * has just got a big task running since a long sleep period. At the same time
>> > + * however it preserves the benefits of the "blocked load" in describing the
>> > + * potential for other tasks waking up on the same CPU.
>> > + *
>> > + * Return: the estimated utlization for the specified CPU
>> > + */
>> > +static inline unsigned long cpu_util_est(int cpu)
>> > +{
>> > +       struct sched_avg *sa = &cpu_rq(cpu)->cfs.avg;
>> > +       unsigned long util = cpu_util(cpu);
>> > +
>> > +       if (!sched_feat(UTIL_EST))
>> > +               return util;
>> > +
>> > +       return max(util, util_est(sa, UTIL_EST_LAST));
>> > +}
>> > +
>> >  static inline int task_util(struct task_struct *p)
>> >  {
>> >         return p->se.avg.util_avg;
>> > @@ -6007,11 +6033,19 @@ static int cpu_util_wake(int cpu, struct task_struct *p)
>> >
>> >         /* Task has no contribution or is new */
>> >         if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
>> > -               return cpu_util(cpu);
>> > +               return cpu_util_est(cpu);
>> >
>> >         capacity = capacity_orig_of(cpu);
>> >         util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
>> >
>> > +       /*
>> > +        * Estimated utilization tracks only tasks already enqueued, but still
>> > +        * sometimes can return a bigger value than PELT, for example when the
>> > +        * blocked load is negligible wrt the estimated utilization of the
>> > +        * already enqueued tasks.
>> > +        */
>> > +       util = max_t(long, util, cpu_util_est(cpu));
>> > +
>>
>> We are supposed to discount the task's util from its CPU. But the
>> cpu_util_est() can potentially return cpu_util() which includes the
>> task's utilization.
>
> You right, this instead should cover all the cases:
>
> ---8<---
>  static int cpu_util_wake(int cpu, struct task_struct *p)
>  {
> -       unsigned long util, capacity;
> +       unsigned long util_est = cpu_util_est(cpu);
> +       unsigned long capacity;
>
>         /* Task has no contribution or is new */
>         if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
> -               return cpu_util(cpu);
> +               return util_est;
>
>         capacity = capacity_orig_of(cpu);
> -       util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
> +       if (cpu_util(cpu) > util_est)
> +               util = max_t(long, cpu_util(cpu) - task_util(p), 0);
> +       else
> +               util = util_est;
>
>         return (util >= capacity) ? capacity : util;
>  }
> ---8<---
>
> Indeed:
>
> - if *p is the only task sleeping on that CPU, then:
>       (cpu_util == task_util) > (cpu_util_est == 0)
>   and thus we return:
>       (cpu_util - task_util) == 0
>
> - if other tasks are SLEEPING on the same CPU, which however is IDLE, then:
>       cpu_util > (cpu_util_est == 0)
>   and thus we discount *p's blocked load by returning:
>       (cpu_util - task_util) >= 0
>
> - if other tasks are RUNNABLE on that CPU and
>       (cpu_util_est > cpu_util)
>   then we wanna use cpu_util_est since it returns a more restrictive
>   estimation of the spare capacity on that CPU, by just considering
>   the expected utilization of tasks already runnable on that CPU.
>
> What do you think?
>

This looks good to me.

Thanks,
Pavan

-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
Linux Foundation Collaborative Project

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