Re: [RFC PATCH 3/7] sched/fair: Remove magic margin in fits_capacity()

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 28/08/2023 01:31, Qais Yousef wrote:
> 80% margin is a magic value that has served its purpose for now, but it
> no longer fits the variety of systems exist today. If a system is over
> powered specifically, this 80% will mean we leave a lot of capacity
> unused before we decide to upmigrate on HMP system.
> 
> The upmigration behavior should rely on the fact that a bad decision
> made will need load balance to kick in to perform misfit migration. And
> I think this is an adequate definition for what to consider as enough
> headroom to consider whether a util fits capacity or not.
> 
> Use the new approximate_util_avg() function to predict the util if the
> task continues to run for TICK_US. If the value is not strictly less
> than the capacity, then it must not be placed there, ie considered
> misfit.
> 
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>  kernel/sched/fair.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0b7445cd5af9..facbf3eb7141 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -109,16 +109,31 @@ int __weak arch_asym_cpu_priority(int cpu)
>  }
>  
>  /*
> - * The margin used when comparing utilization with CPU capacity.
> + * The util will fit the capacity if it has enough headroom to grow within the
> + * next tick - which is when any load balancing activity happens to do the
> + * correction.
>   *
> - * (default: ~20%)
> + * If util stays within the capacity before tick has elapsed, then it should be
> + * fine. If not, then a correction action must happen shortly after it starts
> + * running, hence we treat it as !fit.
> + *
> + * TODO: TICK is not actually accurate enough. balance_interval is the correct
> + * one to use as the next load balance doesn't not happen religiously at tick.
> + * Accessing balance_interval might be tricky and will require some refactoring
> + * first.
>   */

I understand that you want to have a more intelligent margin (depending
on the util value) but why you want to use the time value of TICK_USEC
(or the balance_interval)?

We call fits_capacity() e.g. in wakeup and the next lb can just happen
immediately after it.

> -#define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)
> +static inline bool fits_capacity(unsigned long util, unsigned long capacity)
> +{
> +	return approximate_util_avg(util, TICK_USEC) < capacity;
> +}

[...]