Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function

Quentin Perret
2018-03-21T14:02:37Z

On Wednesday 21 Mar 2018 at 12:26:21 (+0000), Patrick Bellasi wrote:
> On 21-Mar 10:04, Juri Lelli wrote:
> > Hi,
> >
> > On 20/03/18 09:43, Dietmar Eggemann wrote:
> > > From: Quentin Perret <quentin.perret@arm.com>
> > >
> > > In preparation for the definition of an energy-aware wakeup path, a
> > > helper function is provided to estimate the consequence on system energy
> > > when a specific task wakes-up on a specific CPU. compute_energy()
> > > estimates the OPPs to be reached by all frequency domains and estimates
> > > the consumption of each online CPU according to its energy model and its
> > > percentage of busy time.
> > >
> > > 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 | 81 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 81 insertions(+)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 6c72a5e7b1b0..76bd46502486 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6409,6 +6409,30 @@ static inline int cpu_overutilized(int cpu)
> > > }
> > >
> > > /*
> > > + * Returns the util of "cpu" if "p" wakes up on "dst_cpu".
> > > + */
> > > +static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
> > > +{
> > > + unsigned long util = cpu_rq(cpu)->cfs.avg.util_avg;
> >
> > What about other classes? Shouldn't we now also take into account
> > DEADLINE (as schedutil does)?
>
> Good point, although that would likely require to factor out from
> schedutil the utilization aggregation function, isn't it?
>
> > BTW, we now also have a getter method in sched/sched.h; it takes
> > UTIL_EST into account, though. Do we need to take that into account when
> > estimating energy consumption?
>
> Actually I think that this whole function can be written "just" as:
>
> ---8<---
> unsigned long util = cpu_util_wake(cpu);
>
> if (cpu != dst_cpu)
> return util;
>
> return min(util + task_util(p), capacity_orig_of(cpu));
> ---8<---
>

Yes this should be functionally equivalent. However, with your
suggestion you can potentially remove the task contribution from the
cpu_util in cpu_util_wake() and then add it back right after if
cpu==dst_cpu. This is sub-optimal and that's why I implemented things
slightly differently. But maybe this optimization really is too small to
justify the extra complexity involved ...

> which will reuse existing functions as well as getting for free other
> stuff (like the CPU util_est).
>
> Considering your observation above, it makes also easy to add into
> util the DEADLINE and RT utilizations, just before returning the
> value.
>
> > > + unsigned long capacity = capacity_orig_of(cpu);
> > > +
> > > + /*
> > > + * If p is where it should be, or if it has no impact on cpu, there is
> > > + * not much to do.
> > > + */
> > > + if ((task_cpu(p) == dst_cpu) || (cpu != task_cpu(p) && cpu != dst_cpu))
> > > + goto clamp_util;
> > > +
> > > + if (dst_cpu == cpu)
> > > + util += task_util(p);
> > > + else
> > > + util = max_t(long, util - task_util(p), 0);
> > > +
> > > +clamp_util:
> > > + return (util >= capacity) ? capacity : util;
> > > +}
> > > +
> > > +/*
> > > * Disable WAKE_AFFINE in the case where task @p doesn't fit in the
> > > * capacity of either the waking CPU @cpu or the previous CPU @prev_cpu.
> > > *
> > > @@ -6432,6 +6456,63 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> > > return !util_fits_capacity(task_util(p), min_cap);
> > > }
> > >
> > > +static struct capacity_state *find_cap_state(int cpu, unsigned long util)
> > > +{
> > > + struct sched_energy_model *em = *per_cpu_ptr(energy_model, cpu);
> > > + struct capacity_state *cs = NULL;
> > > + int i;
> > > +
> > > + /*
> > > + * As the goal is to estimate the OPP reached for a specific util
> > > + * value, mimic the behaviour of schedutil with a 1.25 coefficient
> > > + */
> > > + util += util >> 2;
> >
> > What about other governors (ondemand for example). Is this supposed to
> > work only when schedutil is in use (if so we should probably make it
> > conditional on that)?
>
> Yes, I would say that EAS mostly makes sense when you have a "minimum"
> control on OPPs... otherwise all the energy estimations are really
> fuzzy.
>
> > Also, even when schedutil is in use, shouldn't we ask it for a util
> > "computation" instead of replicating its _current_ heuristic?
>
> Are you proposing to have the 1.25 factor only here and remove it from
> schedutil?
>
> > I fear the two might diverge in the future.
>
> That could be avoided by factoring out from schedutil the
> "compensation" factor into a proper function to be used by all the
> interested playes, isn't it?
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi