Messages in this thread | | | Date | Thu, 25 Feb 2021 16:23:46 +0000 | From | Vincent Donnefort <> | Subject | Re: [PATCH v2 1/2] sched/fair: Fix task utilization accountability in compute_energy() |
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On Thu, Feb 25, 2021 at 12:45:06PM +0100, Dietmar Eggemann wrote: > On 25/02/2021 09:36, vincent.donnefort@arm.com wrote: > > From: Vincent Donnefort <vincent.donnefort@arm.com> > > [...] > > > 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. > > [...]
Yes exactly. I discovered this issue in a trace where an overutilized happened. Many tasks were migrated to the biggest CPU, but once EAS was back on, it was too late. The big CPU had a high util_avg and the task_util _task_util_est unfairness kept placing tasks on that one, despite being inefficient. All the tasks enqueued on that CPU were enough to keep util_avg high enough and that situation wasn't resolving.
> > > 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 ?
I meant @cpu. When dst_cpu == cpu, it means that we simulate the task being placed on cpu. That's what I wanted to highlight. But I can remove it if you think this is not necessary.
> > 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@arm.com>
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