Messages in this thread Patch in this message | | | From | Dietmar Eggemann <> | Subject | Re: [PATCH 3/4] sched/fair: Remove task_util from effective utilization in feec() | Date | Tue, 4 Jan 2022 18:27:34 +0100 |
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On 09/12/2021 17:11, Vincent Donnefort wrote: > The energy estimation in find_energy_efficient_cpu() (feec()) relies on > the computation of the effective utilization for each CPU of a perf domain > (PD). This effective utilization is then used as an estimation of the busy > time for this pd. The function effective_cpu_util() which gives this value, > scales the utilization relative to IRQ pressure on the CPU to take into > account that the IRQ time is hidden from the task clock. The IRQ scaling is > as follow: > > effective_cpu_util = irq + (cpu_cap - irq)/cpu_cap * util
effective_cpu_util stands for cpu_util (effective_cpu_util(..., ENERGY_UTIL, ...), right? (1)
> Where util is the sum of CFS/RT/DL utilization, cpu_cap the capacity of > the CPU and irq the IRQ avg time. > > If now we take as an example a task placement which doesn't raise the OPP > on the candidate CPU, we can write the energy delta as: > > delta = OPPcost/cpu_cap * (effective_cpu_util(cpu_util + task_util) - > effective_cpu_util(cpu_util)) > = OPPcost/cpu_cap * (cpu_cap - irq)/cpu_cap * task_util > > We end-up with an energy delta depending on the IRQ avg time, which is a > problem: first the time spent on IRQs by a CPU has no effect on the > additional energy that would be consumed by a task. Second, we don't want > to favour a CPU with a higher IRQ avg time value. > > Nonetheless, we need to take the IRQ avg time into account. If a task > placement raises the PD's frequency, it will increase the energy cost for > the entire time where the CPU is busy. A solution is to only use > effective_cpu_util() with the CPU contribution part. The task contribution > is added separately and scaled according to prev_cpu's IRQ time.
This whole idea looks like a follow-up of commit 0372e1cf70c2 ("sched/fair: Fix task utilization accountability in compute_energy()").
I forgot why we still use cpu_util_next(cpu, p, dst_cpu) for FREQUENCY_UTIL though?
> No change for the FREQUENCY_UTIL component of the energy estimation. We
OK, it indirectly says so. (1)
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> @@ -6599,23 +6599,83 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu) > return min(util, capacity_orig_of(cpu)); > } > > +/* > + * Compute the task busy time for compute_energy(). This time cannot be > + * injected directly into effective_cpu_util() because of the IRQ scaling. > + * The latter only makes sense with the most recent CPUs where the task has > + * run. > + */ > +static inline unsigned long > +task_busy_time(struct task_struct *p, int prev_cpu) > +{ > + unsigned long cpu_cap = arch_scale_cpu_capacity(prev_cpu);
s/cpu_cap/max_cap ... to stay consistent
> + unsigned long irq = cpu_util_irq(cpu_rq(prev_cpu));
What about irq >= max_cap ?
effective_cpu_util() has the following condition:
if (unlikely(irq >= max_cap)) return max_cap;
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> + * The contribution of the task @p for which we want to estimate the energy > + * cost is removed (by cpu_util_next()) and must be compted separalyte (see
s/compted separalyte/calculated separately ?
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> +static inline unsigned long > +pd_task_busy_time(struct task_struct *p, struct perf_domain *pd, > + unsigned long cpu_cap, unsigned long tsk_busy_time, > + unsigned long *pd_tsk_busy_time) > +{ > + unsigned long max_cap, pd_cap = 0, pd_busy_time = 0; > + struct cpumask *pd_mask = perf_domain_span(pd); > + int cpu; > + > + max_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
In case we use 'sched, drivers: Remove max param from effective_cpu_util()/sched_cpu_util()'
https://gitlab.arm.com/linux-arm/linux-de/-/commit/150e753e861285e82e9d7c593f1f26075c34e124
we would not have to have max_cap for effective_cpu_util(). This would make the code easier to understand since we already have to pass pd_cap and cpu_cap (cpu_thermal_cap) now.
> + > + /* see compute_energy() */ > + for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
Somehow unrelated ... We now use cpu_online_mask 3 times per PD to iterate over the CPUs. cpu_online_mask can change during the run-queue selection.
We could reuse `select_idle_mask` to create one cpumask at the beginning of feec() and pass it down the fucntions:
See `sched/fair: Rename select_idle_mask to select_rq_mask` and `sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu()`
https://gitlab.arm.com/linux-arm/linux-de/-/commit/ec5bc27a298dd1352dbaff5809743128fa351075
https://gitlab.arm.com/linux-arm/linux-de/-/commit/f73b19968a65b07b0ad5bd1dff721ed1a675a24b
> + unsigned long util = cpu_util_next(cpu, p, -1); > + > + pd_cap += cpu_cap; > + pd_busy_time += effective_cpu_util(cpu, util, max_cap, > + ENERGY_UTIL, NULL); > + } > + > + pd_busy_time = min(pd_cap, pd_busy_time); > + *pd_tsk_busy_time = min(pd_cap, pd_busy_time + tsk_busy_time);
We do `sum_util += min(cpu_util, _cpu_cap (cpu_thermal_cap))` in compute_energy() so far but now you sum up PD capacity (pd_cap) first and then compare the sum_util (i.e. pd_busy_time) with pd_cap. Why?
cpu_util = effective_cpu_util(..., FREQUENCY_UTIL, ...) still uses min(cpu_util, _cpu_cap).
> + > + return pd_busy_time;
This function seems to be a little bit overloaded. It's called pd_task_busy_time but returns `pd` busy time and `pd + task` busy time.
In case we could calculate pd_cap pd_task_busy_time() then this function can only return pd_busy_time and pd_tsk_busy_time could also calculate outside the function.
See `XXX: Calculate pd_cap & pd_tsk_busy_time outside pd_task_busy_time()`
https://gitlab.arm.com/linux-arm/linux-de/-/commit/4512d681fe32eb5e2862c4c5d5a03b1d84129e26
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> @@ -6628,34 +6688,11 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) > */ > for_each_cpu_and(cpu, pd_mask, cpu_online_mask) { > unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu); > - unsigned long cpu_util, util_running = util_freq; > + unsigned long cpu_util; > struct task_struct *tsk = NULL; > > - /* > - * When @p is placed on @cpu: > - * > - * util_running = max(cpu_util, cpu_util_est) + > - * max(task_util, _task_util_est) > - * > - * while cpu_util_next is: max(cpu_util + task_util, > - * cpu_util_est + _task_util_est) > - */ > - if (cpu == dst_cpu) { > + if (cpu == dst_cpu) > tsk = p; > - util_running = > - cpu_util_next(cpu, p, -1) + task_util_est(p); > - } > - > - /* > - * Busy time computation: utilization clamping is not > - * required since the ratio (sum_util / cpu_capacity) > - * is already enough to scale the EM reported power > - * consumption at the (eventually clamped) cpu_capacity. > - */ > - cpu_util = effective_cpu_util(cpu, util_running, cpu_cap, > - ENERGY_UTIL, NULL); > - > - sum_util += min(cpu_util, _cpu_cap); > > /* > * Performance domain frequency: utilization clamping > @@ -6664,12 +6701,12 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) > * NOTE: in case RT tasks are running, by default the > * FREQUENCY_UTIL's utilization can be max OPP. > */ > - cpu_util = effective_cpu_util(cpu, util_freq, cpu_cap, > + cpu_util = effective_cpu_util(cpu, util_freq, max_cap, > FREQUENCY_UTIL, tsk); > - max_util = max(max_util, min(cpu_util, _cpu_cap)); > + max_util = max(max_util, min(cpu_util, cpu_cap)); > }
It's hard to understand since it is unbalanced that `busy time` is calculated in pd_busy_time() whereas `max_util` is still calculated in compute_energy().
IMHO it would be much easier to understand if there would be a pd_max_util() as well so that we could do:
busy_time = get_pd_busy_time() max_util = get_pd_max_util(..., -1, ...) base_energy = compute_energy(..., max_util, busy_time, ...)
busy_time = min(busy_time + tsk_busy_time, pd_cap); if (compute_prev_delta) max_util = get_pd_max_util(..., prev_cpu, ...) prev_delta = compute_energy(..., max_util, busy_time, ...) ... See `XXX: Split get_pd_max_util() from compute_energy()`
https://gitlab.arm.com/linux-arm/linux-de/-/commit/6d79929ee7c8675a127158187786dd1d6b6dd355
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> @@ -6783,13 +6824,27 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > if (max_spare_cap_cpu < 0 && !compute_prev_delta) > continue; > > + /* Account thermal pressure for the energy estimation */ > + cpu_thermal_cap = arch_scale_cpu_capacity( > + cpumask_first(perf_domain_span(pd))); > + cpu_thermal_cap -= arch_scale_thermal_pressure( > + cpumask_first(perf_domain_span(pd)));
Yes, we should calculate cpu_thermal_cap only once per PD. Can you make this a little bit more easy to read by getting `cpu = cpumask_first(perf_domain_span(pd));` first?
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