Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

[Vincent Guittot]

On 7 September 2015 at 17:37, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> On 04/09/15 00:51, Steve Muckle wrote:
>> Hi Morten, Dietmar,
>>
>> On 08/14/2015 09:23 AM, Morten Rasmussen wrote:
>> ...
>>> + * cfs_rq.avg.util_avg is the sum of running time of runnable tasks plus the
>>> + * recent utilization of currently non-runnable tasks on a CPU. It represents
>>> + * the amount of utilization of a CPU in the range [0..capacity_orig] where
>>
>> I see util_sum is scaled by SCHED_LOAD_SHIFT at the end of
>> __update_load_avg(). If there is now an assumption that util_avg may be
>> used directly as a capacity value, should it be changed to
>> SCHED_CAPACITY_SHIFT? These are equal right now, not sure if they will
>> always be or if they can be combined.
>
> You're referring to the code line
>
> 2647   sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX;
>
> in __update_load_avg()?
>
> Here we actually scale by 'SCHED_LOAD_SCALE/LOAD_AVG_MAX' so both values are
> load related.

I agree with Steve that there is an issue from a unit point of view

sa->util_sum and LOAD_AVG_MAX have the same unit so sa->util_avg is a
load because of << SCHED_LOAD_SHIFT)

Before this patch , the translation from load to capacity unit was
done in get_cpu_usage with "* capacity) >> SCHED_LOAD_SHIFT"

So you still have to change the unit from load to capacity with a "/
SCHED_LOAD_SCALE * SCHED_CAPACITY_SCALE" somewhere.

sa->util_avg = ((sa->util_sum << SCHED_LOAD_SHIFT) /SCHED_LOAD_SCALE *
SCHED_CAPACITY_SCALE / LOAD_AVG_MAX = (sa->util_sum <<
SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX;


Regards,
Vincent


>
> LOAD (UTIL) and CAPACITY have the same SCALE and SHIFT values because
> SCHED_LOAD_RESOLUTION is always defined to 0. scale_load() and
> scale_load_down() are also NOPs so this area is probably
> worth a separate clean-up.
> Beyond that, I'm not sure if the current functionality is
> broken if we use different SCALE and SHIFT values for LOAD and CAPACITY?
>
>>
>>> + * capacity_orig is the cpu_capacity available at * the highest frequency
>>
>> spurious *
>>
>> thanks,
>> Steve
>>
>
> Fixed.
>
> Thanks,
>
> -- Dietmar
>
> -- >8 --
>
> From: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Date: Fri, 14 Aug 2015 17:23:13 +0100
> Subject: [PATCH] sched/fair: Get rid of scaling utilization by capacity_orig
>
> Utilization is currently scaled by capacity_orig, but since we now have
> frequency and cpu invariant cfs_rq.avg.util_avg, frequency and cpu scaling
> now happens as part of the utilization tracking itself.
> So cfs_rq.avg.util_avg should no longer be scaled in cpu_util().
>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> ---
>  kernel/sched/fair.c | 38 ++++++++++++++++++++++----------------
>  1 file changed, 22 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2074d45a67c2..a73ece2372f5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4824,33 +4824,39 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  done:
>         return target;
>  }
> +
>  /*
>   * cpu_util returns the amount of capacity of a CPU that is used by CFS
>   * tasks. The unit of the return value must be the one of capacity so we can
>   * compare the utilization with the capacity of the CPU that is available for
>   * CFS task (ie cpu_capacity).
> - * cfs.avg.util_avg is the sum of running time of runnable tasks on a
> - * CPU. It represents the amount of utilization of a CPU in the range
> - * [0..SCHED_LOAD_SCALE]. The utilization of a CPU can't be higher than the
> - * full capacity of the CPU because it's about the running time on this CPU.
> - * Nevertheless, cfs.avg.util_avg can be higher than SCHED_LOAD_SCALE
> - * because of unfortunate rounding in util_avg or just
> - * after migrating tasks until the average stabilizes with the new running
> - * time. So we need to check that the utilization stays into the range
> - * [0..cpu_capacity_orig] and cap if necessary.
> - * Without capping the utilization, a group could be seen as overloaded (CPU0
> - * utilization at 121% + CPU1 utilization at 80%) whereas CPU1 has 20% of
> - * available capacity.
> + *
> + * cfs_rq.avg.util_avg is the sum of running time of runnable tasks plus the
> + * recent utilization of currently non-runnable tasks on a CPU. It represents
> + * the amount of utilization of a CPU in the range [0..capacity_orig] where
> + * capacity_orig is the cpu_capacity available at the highest frequency
> + * (arch_scale_freq_capacity()).
> + * The utilization of a CPU converges towards a sum equal to or less than the
> + * current capacity (capacity_curr <= capacity_orig) of the CPU because it is
> + * the running time on this CPU scaled by capacity_curr.
> + *
> + * Nevertheless, cfs_rq.avg.util_avg can be higher than capacity_curr or even
> + * higher than capacity_orig because of unfortunate rounding in
> + * cfs.avg.util_avg or just after migrating tasks and new task wakeups until
> + * the average stabilizes with the new running time. We need to check that the
> + * utilization stays within the range of [0..capacity_orig] and cap it if
> + * necessary. Without utilization capping, a group could be seen as overloaded
> + * (CPU0 utilization at 121% + CPU1 utilization at 80%) whereas CPU1 has 20% of
> + * available capacity. We allow utilization to overshoot capacity_curr (but not
> + * capacity_orig) as it useful for predicting the capacity required after task
> + * migrations (scheduler-driven DVFS).
>   */
>  static int cpu_util(int cpu)
>  {
>         unsigned long util = cpu_rq(cpu)->cfs.avg.util_avg;
>         unsigned long capacity = capacity_orig_of(cpu);
>
> -       if (util >= SCHED_LOAD_SCALE)
> -               return capacity;
> -
> -       return (util * capacity) >> SCHED_LOAD_SHIFT;
> +       return (util >= capacity) ? capacity : util;
>  }
>
>  /*
> --
> 1.9.1
>
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