Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function

[Juri Lelli <juri.lelli@gmail.com>]

Hi,

On 20/03/18 09:43, Dietmar Eggemann wrote:
> From: Quentin Perret <quentin.perret@arm.com>
> 
> In preparation for the definition of an energy-aware wakeup path, a
> helper function is provided to estimate the consequence on system energy
> when a specific task wakes-up on a specific CPU. compute_energy()
> estimates the OPPs to be reached by all frequency domains and estimates
> the consumption of each online CPU according to its energy model and its
> percentage of busy time.
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Quentin Perret <quentin.perret@arm.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  kernel/sched/fair.c | 81 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6c72a5e7b1b0..76bd46502486 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6409,6 +6409,30 @@ static inline int cpu_overutilized(int cpu)
>  }
>  
>  /*
> + * Returns the util of "cpu" if "p" wakes up on "dst_cpu".
> + */
> +static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
> +{
> +	unsigned long util = cpu_rq(cpu)->cfs.avg.util_avg;

What about other classes? Shouldn't we now also take into account
DEADLINE (as schedutil does)?

BTW, we now also have a getter method in sched/sched.h; it takes
UTIL_EST into account, though. Do we need to take that into account when
estimating energy consumption?

> +	unsigned long capacity = capacity_orig_of(cpu);
> +
> +	/*
> +	 * If p is where it should be, or if it has no impact on cpu, there is
> +	 * not much to do.
> +	 */
> +	if ((task_cpu(p) == dst_cpu) || (cpu != task_cpu(p) && cpu != dst_cpu))
> +		goto clamp_util;
> +
> +	if (dst_cpu == cpu)
> +		util += task_util(p);
> +	else
> +		util = max_t(long, util - task_util(p), 0);
> +
> +clamp_util:
> +	return (util >= capacity) ? capacity : util;
> +}
> +
> +/*
>   * Disable WAKE_AFFINE in the case where task @p doesn't fit in the
>   * capacity of either the waking CPU @cpu or the previous CPU @prev_cpu.
>   *
> @@ -6432,6 +6456,63 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
>  	return !util_fits_capacity(task_util(p), min_cap);
>  }
>  
> +static struct capacity_state *find_cap_state(int cpu, unsigned long util)
> +{
> +	struct sched_energy_model *em = *per_cpu_ptr(energy_model, cpu);
> +	struct capacity_state *cs = NULL;
> +	int i;
> +
> +	/*
> +	 * As the goal is to estimate the OPP reached for a specific util
> +	 * value, mimic the behaviour of schedutil with a 1.25 coefficient
> +	 */
> +	util += util >> 2;

What about other governors (ondemand for example). Is this supposed to
work only when schedutil is in use (if so we should probably make it
conditional on that)?

Also, even when schedutil is in use, shouldn't we ask it for a util
"computation" instead of replicating its _current_ heuristic? I fear
the two might diverge in the future.

Best,

- Juri