On Wednesday 21 Mar 2018 at 15:35:18 (+0000), Patrick Bellasi wrote: > On 20-Mar 09:43, Dietmar Eggemann wrote: > > From: Quentin Perret <quentin.per...@arm.com> > > [...] > > > 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)); > ^^^^^^^^^^^^^^^^^^^^^ > > Since here we are at scheduling CFS tasks, should we not better use > capacity_of() to account for RT/IRQ pressure ?
Yes, definitely. I change this in v2. > > > +} > > + > > +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)) > > We are still waking up a task... what if the task was previously > running on a big CPU which is now idle? > > I understand that from a _relative_ energy_diff standpoint there is > not much to do for a 0 utilization task. However, for those tasks we > can still try to return the most energy efficient CPU among the ones > in their cpus_allowed mask. > > It should be a relatively low overhead (maybe contained in a fallback > most_energy_efficient_cpu() kind of function) which allows, for > example on ARM big.LITTLE systems, to consolidate those tasks on > LITTLE CPUs instead for example keep running them on a big CPU. Hmmmm so the difficult thing about a task with 0 util is that you don't know if this is really a small task, or a big task with a very long period. The only useful thing you know for sure about the task is where it ran last time, so I guess that makes sense to use that information rather than make assumptions. There is no perfect solution using the util_avg of the task. Now, UTIL_EST is changing the game here. If we use it for task placement (which I think is the right thing to do), this issue should be a lot easier to solve. What do you think ? Thanks, Quentin