Re: [PATCH] sched/fair: use signed long when compute energy delta in eas

[Xuewen Yan <xuewen.yan94@gmail.com>]

Hi Dietmar

On Fri, Apr 2, 2021 at 2:07 AM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> +cc: Pierre Gondois <pierre.gondois@arm.com>
>
> On 30/03/2021 11:45, Quentin Perret wrote:
> > Hi,
> >
> > On Tuesday 30 Mar 2021 at 13:21:54 (+0800), Xuewen Yan wrote:
> >> From: Xuewen Yan <xuewen.yan@unisoc.com>
> >>
> >> now the energy delta compute as follow:
> >>
> >> base_energy_pd = compute_energy(p, -1, pd);
> >>      --->Traverse all CPUs in pd
> >>      --->em_pd_energy()
> >> ----------------------------------------------------- \
> >> search for the max_sapre_cap_cpu                       \
> >> ---------------------------------                       search time
> >> cur_delta = compute_energy(p, max_spare_cap_cpu, pd);  /
> >>      --->Traverse all CPUs in pd                   /
> >> ---------------------------------------------------- /
> >>      --->em_pd_energy()
> >> cur_delta -= base_energy_pd;
> >>
> >> During the search_time, or when calculate the cpu_util in
> >> compute_energy(), there may occurred task dequeue or cpu_util change,
> >> it may cause the cur_energy < base_energy_pd, so the cur_delta
> >> would be negative. But the cur_delta is unsigned long, at this time,
> >> the cur_delta would always bigger than best_delta of last pd.
> >>
> >> Change the vars to signed long.
> >
> > Is that really helping though?
> >
> > Yes you will not overflow, but the decision is still 'wrong' if the util
> > values are not stable for the entire wake-up. I think folks on the Arm
> > side had patches to try and cache the util values upfront, and then use
> > them throughout feec() and compute_energy(), which I think would be a
> > better fix.
> >
> > Dietmar, wdyt?
>
> Yes, we have some patches from Pierre Gondois which introduce a pd cache
> to store the CPU utilization values so they can be reused for 'cpu !=
> dst_cpu' calculations within find_energy_efficient_cpu() (feec()).
>
> We did run them in our Jan 2021 EAS integration:
>
> https://gitlab.arm.com/linux-arm/linux-power/-/commits/eas/next/integration-20210129
>
>   sched: Allocate pd_cache when EAS is enabled
>   sched/fair: Use pd_cache to speed up find_energy_efficient_cpu()
>
> We haven't posted them since we're still looking for a story to justify
> the extra complexity. The experiments on Arm64 Juno (2 big, 4 little
> CPUs) showed 1-2% failure due to changes of CPU utilization values
> during feec(). There was a 5% (big CPU)-10% (little CPU) runtime
> reduction for feec() with the patches.

I test the patch, but the overflow still exists.
In the "sched/fair: Use pd_cache to speed up find_energy_efficient_cpu()"
I wonder why recompute the cpu util when cpu==dst_cpu in compute_energy(),
when the dst_cpu's util change, it also would cause the overflow.

Thanks