Re: [RFC PATCH 6/7] sched/fair: update cpu_capcity to reflect thermal pressure

[Javi Merino <javi.merino@kernel.org>]

On Tue, Oct 09, 2018 at 12:25:01PM -0400, Thara Gopinath wrote:
> cpu_capacity relflects the maximum available capacity of a cpu. Thermal
> pressure on a cpu means this maximum available capacity is reduced. This
> patch reduces the average thermal pressure for a cpu from its maximum
> available capacity so that cpu_capacity reflects the actual
> available capacity.
> 
> Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
> ---
>  kernel/sched/fair.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7deb1d0..8651e55 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7497,6 +7497,7 @@ static unsigned long scale_rt_capacity(int cpu)
>  
>  	used = READ_ONCE(rq->avg_rt.util_avg);
>  	used += READ_ONCE(rq->avg_dl.util_avg);
> +	used += READ_ONCE(rq->avg_thermal.load_avg);

IIUIC, you are treating thermal pressure as an artificial load on the
cpu.  If so, this sounds like a hard to maintain hack.  Thermal
pressure have different characteristics to utilization.  What happens
if thermal sets the cpu cooling state back to 0 because there is
thermal headroom again?  Do we keep adding this artificial load to the
cpu just because there was thermal pressure in the past and let it
decay as if it was cpu load?

Cheers,
Javi