Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

Joel Fernandes
2018-03-24T00:36:13Z

On Fri, Mar 23, 2018 at 9:00 AM, Morten Rasmussen
<morten.rasmussen@arm.com> wrote:
> On Thu, Mar 22, 2018 at 09:27:43AM -0700, Joel Fernandes wrote:
>> >
>> > In case an energy model is available, waking tasks are re-routed into a
>> > new energy-aware placement algorithm. The eligible CPUs to be used in the
>> > energy-aware wakeup path are restricted to the highest non-overutilized
>> > sched_domain containing prev_cpu and this_cpu. If no such domain is found,
>> > the tasks go through the usual wake-up path, hence energy-aware placement
>> > happens only in lightly utilized scenarios.
>> >
>> > The selection of the most energy-efficient CPU for a task is achieved by
>> > estimating the impact on system-level active energy resulting from the
>> > placement of the task on each candidate CPU. The best CPU energy-wise is
>> > then selected if it saves a large enough amount of energy with respect to
>> > prev_cpu.
>> >
>> > Although it has already shown significant benefits on some existing
>> > targets, this brute force approach clearly cannot scale to platforms with
>> > numerous CPUs. This patch is an attempt to do something useful as writing
>> > a fast heuristic that performs reasonably well on a broad spectrum of
>> > architectures isn't an easy task. As a consequence, the scope of usability
>> > of the energy-aware wake-up path is restricted to systems with the
>> > SD_ASYM_CPUCAPACITY flag set. These systems not only show the most
>> > promising opportunities for saving energy but also typically feature a
>> > limited number of logical CPUs.
>> >
>> > Cc: Ingo Molnar <mingo@redhat.com>
>> > Cc: Peter Zijlstra <peterz@infradead.org>
>> > Signed-off-by: Quentin Perret <quentin.perret@arm.com>
>> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> > ---
>> > kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>> > 1 file changed, 71 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index 76bd46502486..65a1bead0773 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -6513,6 +6513,60 @@ static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
>> > return energy;
>> > }
>> >
>> > +static bool task_fits(struct task_struct *p, int cpu)
>> > +{
>> > + unsigned long next_util = cpu_util_next(cpu, p, cpu);
>> > +
>> > + return util_fits_capacity(next_util, capacity_orig_of(cpu));
>> > +}
>> > +
>> > +static int find_energy_efficient_cpu(struct sched_domain *sd,
>> > + struct task_struct *p, int prev_cpu)
>> > +{
>> > + unsigned long cur_energy, prev_energy, best_energy;
>> > + int cpu, best_cpu = prev_cpu;
>> > +
>> > + if (!task_util(p))
>> > + return prev_cpu;
>> > +
>> > + /* Compute the energy impact of leaving the task on prev_cpu. */
>> > + prev_energy = best_energy = compute_energy(p, prev_cpu);
>>
>> Is it possible that before the wakeup, the task's affinity is changed
>> so that p->cpus_allowed no longer contains prev_cpu ? In that case
>> prev_energy wouldn't matter since previous CPU is no longer an option?
>
> It is possible to wake-up with a disallowed prev_cpu. In fact
> select_idle_sibling() may happily return a disallowed cpu in that case.
> The mistake gets fixed in select_task_rq() which uses
> select_fallback_rq() to find an allowed cpu instead.
>
> Could we fix the issue in find_energy_efficient_cpu() by a simple test
> like below
>
> if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
> prev_energy = best_energy = compute_energy(p, prev_cpu);
> else
> prev_energy = best_energy = ULONG_MAX;

Yes, I think setting to ULONG_MAX in this case is Ok with me.

thanks,

- Joel