Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2 1/2] sched/fair: Fix task utilization accountability in compute_energy() | From | Dietmar Eggemann <> | Date | Thu, 25 Feb 2021 12:45:06 +0100 |
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On 25/02/2021 09:36, vincent.donnefort@arm.com wrote: > From: Vincent Donnefort <vincent.donnefort@arm.com>
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> cpu_util_next() estimates the CPU utilization that would happen if the > task was placed on dst_cpu as follows: > > max(cpu_util + task_util, cpu_util_est + _task_util_est) > > The task contribution to the energy delta can then be either: > > (1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0 > and _task_util_est > cpu_util. > (2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est. > > (cpu_util_est doesn't appear here. It is 0 when a CPU is idle and > otherwise must be small enough so that feec() takes the CPU as a > 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 > index 7043bb0f2621..146ac9fec4b6 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6573,8 +6573,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) > * its pd list and will not be accounted by compute_energy(). > */ > for_each_cpu_and(cpu, pd_mask, cpu_online_mask) { > - unsigned long cpu_util, util_cfs = cpu_util_next(cpu, p, dst_cpu); > - struct task_struct *tsk = cpu == dst_cpu ? p : NULL; > + unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu); > + unsigned long cpu_util, util_running = util_freq; > + 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) > + */
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?
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Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
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