Re: [PATCH v2 1/2] sched/fair: Fix task utilization accountability in compute_energy()
From: Dietmar Eggemann
Date: Thu Feb 25 2021 - 06:46:22 EST
On 25/02/2021 09:36, vincent.donnefort@xxxxxxx wrote:
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From: Vincent Donnefort <vincent.donnefort@xxxxxxx>
[...]
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cpu_util_next() estimates the CPU utilization that would happen if the
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task was placed on dst_cpu as follows:
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max(cpu_util + task_util, cpu_util_est + _task_util_est)
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The task contribution to the energy delta can then be either:
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(1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0
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and _task_util_est > cpu_util.
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(2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est.
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(cpu_util_est doesn't appear here. It is 0 when a CPU is idle and
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otherwise must be small enough so that feec() takes the CPU as a
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potential target for the task placement)
I still don't quite get the reasoning for (2) why task_util is used as
task contribution.
So we use 'cpu_util + task_util' instead of 'cpu_util_est +
_task_util_est' in (2).
I.e. since _task_util_est is always >= task_util during wakeup, cpu_util
must be > cpu_util_est (by more than _task_util_est - task_util).
I can see it for a CPU whose cpu_util has a fair amount of contributions
from blocked tasks which cpu_util_est wouldn't have.
[...]
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diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
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index 7043bb0f2621..146ac9fec4b6 100644
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--- a/kernel/sched/fair.c
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+++ b/kernel/sched/fair.c
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@@ -6573,8 +6573,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
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* its pd list and will not be accounted by compute_energy().
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*/
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for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
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- unsigned long cpu_util, util_cfs = cpu_util_next(cpu, p, dst_cpu);
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- struct task_struct *tsk = cpu == dst_cpu ? p : NULL;
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+ unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu);
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+ unsigned long cpu_util, util_running = util_freq;
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+ struct task_struct *tsk = NULL;
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+
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+ /*
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+ * When @p is placed on @cpu:
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+ *
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+ * util_running = max(cpu_util, cpu_util_est) +
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+ * max(task_util, _task_util_est)
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+ *
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+ * while cpu_util_next is: max(cpu_util + task_util,
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+ * cpu_util_est + _task_util_est)
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+ */
Nit pick:
s/on @cpu/on @dst_cpu ?
s/while cpu_util_next is/while cpu_util_next(cpu, p, cpu) would be
If dst_cpu != cpu (including dst_cpu == -1) task_util and _task_util_est
are not added to util resp. util_est.
Not sure if this is clear from the source code here?
[...]
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>