Messages in this thread | | | Subject | Re: [RFC PATCH] sched/fair: use max_spare_cap_cpu if it is more energy efficient | From | Dietmar Eggemann <> | Date | Mon, 25 Oct 2021 15:04:47 +0200 |
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On 22/10/2021 06:05, Xuewen Yan wrote: > Hi Chunguang > > brookxu <brookxu.cn@gmail.com> 于2021年10月21日周四 下午4:24写道: >> >> From: Chunguang Xu <brookxu@tencent.com> >> >> When debugging EAS, I found that if the task is migrated to >> max_spare_cap_cpu, even if the power consumption of pd is lower,
The task p hasn't been migrated yet. `max_spare_cap_cpu` here is only a potential candidate CPU to be selected for p.
>> we still put the task on prev_cpu. Maybe we should fix it. >> >> Signed-off-by: Chunguang Xu <brookxu@tencent.com> >> --- >> kernel/sched/fair.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index ff69f245b939..2ae7e03de6d2 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6867,8 +6867,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) >> /* Evaluate the energy impact of using max_spare_cap_cpu. */ >> if (max_spare_cap_cpu >= 0) { >> cur_delta = compute_energy(p, max_spare_cap_cpu, pd); >> - if (cur_delta < base_energy_pd) > > this is aimed to prevent the cur_delta < 0, and usuallly, when the > task was put on the max_spare_cpu, the cur_power should be bigger than > base_pd_power, > if the cur_power < base_pd_power, the cpu util may have changed, at > this time, we should keep prev_cpu. > > You can look at below discuss and patch: > https://lore.kernel.org/all/20210429101948.31224-3-Pierre.Gondois@arm.com/ > https://lore.kernel.org/all/CAB8ipk_vgtg5d1obH36BYfNLZosbwr2k_U3xnAD4=H5uZt_M_g@mail.gmail.com/
That's correct. `prev_delta < base_energy_pd` or `cur_delta < base_energy_pd` indicate the rare case that `compute_energy() { -> cpu_util_next() -> cpu util }` returns a higher energy value for the perf domain w/o the task p than w/ it.
`base_energy_pd` stands for the energy spend on the CPUs of the Perf Domain (PD) w/o considering the task p (compute_energy(p, *-1*, pd)), `dst_cpu == -1`.
If this happens to a candidate CPU (prev_cpu or a per-PD max_spare_cap_cpu) we bail out and return target (i.e. prev_cpu) because we can't compare the energy values (prev_delta and best_delta) later on in this case.
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