Re: [PATCH] sched/fair: use signed long when compute energy delta in eas

[Xuewen Yan <xuewen.yan94@gmail.com>]

Hi

On Tue, Apr 13, 2021 at 1:15 AM Pierre Gondois <pierre.gondois@arm.com> wrote:
>
> Hi
> > > > >
> > > > > This patch-set is not significantly improving the execution time of
> > > > > feec(). The results we have so far are an improvement of 5-10% in
> > > > > execution time, with feec() being executed in < 10us. So the
> > gain is not
> > > > > spectacular.
> > > >
> > > > well， I meaned to cache all util value and compute energy with
> > caches,
> > > > when
> > > > (cpu==dst_cpu), use caches instead of updating util, and do not get
> > > > util with function:
> > > >  "effective_cpu_util()", to compute util with cache.
> > > > I add more parameters into pd_cache:
> > > > struct pd_cache {
> > > >         unsigned long util;
> > > >         unsigned long util_est;
> > > >         unsigned long util_cfs;
> > > >         unsigned long util_irq;
> > > >         unsigned long util_rt;
> > > >         unsigned long util_dl;
> > > >         unsigned long bw_dl;
> > > >         unsigned long freq_util;
> > > >         unsigned long nrg_util;
> > > > };
> > > > In this way, it can avoid util update while feec. I tested with it,
> > > > and the negative delta disappeared.
> > > > Maybe this is not a good method, but it does work.
> > > If I understand correctly, you put all the fields used by
> > > core.c:effective_cpu_util() in the caches, allowing to have values not
> > > subject to updates.
> > Yes.
> > > core.c:effective_cpu_util() isn't only called from
> > > fair.c:compute_energy(). It is used in the cpufreq_schedutil.c and
> > > cpufreq_cooling.c (through core.c:sched_cpu_util()).
> > > Did you have to duplicate core.c:effective_cpu_util() to have a second
> > > version using the caches ? If yes, I think the function was meant to be
> > > unique so that all the utilization estimations go through the same path.
> > >
> > I defined a new function to distinguish it from the effective_cpu_util.
> >
> > > If your concern is to avoid negative delta, I think just bailing out
> > > when this happens should be sufficient. As shown in the last message,
> > > having a wrong placement should not happen that often, plus the prev_cpu
> > > should be used which should be ok.
> > In your patch, you didn't actually choose the prev_cpu. you return (-1);
> >
> > > If you want to cache the values, I think a stronger justification will
> > > be asked: this seems to be a big modification compared to the initial
> > > issue, knowing that another simpler solution is available (i.e. bailing
> > > out). I was not able to prove there was a significant gain in the
> > > find_energy_efficient_cpu() execution time, but I would be happy if you
> > > can, or if you find other arguments.
> > Yes, you are right, perhaps there is indeed no need for such a big
> > modification.
> >
> > Regards
>
> In fair.c:select_task_rq_fair(), if feec() returns a error (< 0), then
> prev_cpu is selected. I think it's better to still let feec() signal
> that something happened and let select_task_rq_fair() select prev_cpu by
> itself.
In fair.c:select_task_rq_fair(), when feec() returns a error (< 0),
the new_cpu = find_idlest_cpu() or select_idle_sibling().
In your patch, you should return prev_cpu instead of -1 if you want to
select the prev_cpu.

> Are you planning to submit a V2 with the bail out mechanism ?
Maybe it would be better if you submitted the V2 ? I just check the
patch and raised some questions.
>
Regards