Messages in this thread | | | Date | Fri, 23 Mar 2018 16:00:59 +0000 | From | Morten Rasmussen <> | Subject | Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up |
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On Thu, Mar 22, 2018 at 09:27:43AM -0700, Joel Fernandes wrote: > Hi, > > On Tue, Mar 20, 2018 at 2:43 AM, Dietmar Eggemann > <dietmar.eggemann@arm.com> wrote: > > > > From: Quentin Perret <quentin.perret@arm.com> > > > > 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;
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