Messages in this thread | | | From | Joel Fernandes <> | Date | Fri, 23 Mar 2018 17:36:13 -0700 | Subject | Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up |
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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
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