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

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

+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.