Messages in this thread | | | Subject | Re: [PATCH] sched/fair: use signed long when compute energy delta in eas | From | Dietmar Eggemann <> | Date | Thu, 1 Apr 2021 20:07:34 +0200 |
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+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.
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