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

[Pierre <pierre.gondois@arm.com>]

Hi,
> Hi
>
> On Wed, Apr 7, 2021 at 10:11 PM Pierre <pierre.gondois@arm.com> wrote:
> >
> > Hi,
> > > I test the patch, but the overflow still exists.
> > > In the "sched/fair: Use pd_cache to speed up 
> find_energy_efficient_cpu()"
> > > I wonder why recompute the cpu util when cpu==dst_cpu in 
> compute_energy(),
> > > when the dst_cpu's util change, it also would cause the overflow.
> >
> > The patches aim to cache the energy values for the CPUs whose
> > utilization is not modified (so we don't have to compute it multiple
> > times). The values cached are the 'base values' of the CPUs, i.e. when
> > the task is not placed on the CPU. When (cpu==dst_cpu) in
> > compute_energy(), it means the energy values need to be updated instead
> > of using the cached ones.
> >
> well, is it better to use the task_util(p) + cache values ? but in
> this case, the cache
> values may need more parameters.

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.

>
> > You are right, there is still a possibility to have a negative delta
> > with the patches at:
> > 
> https://gitlab.arm.com/linux-arm/linux-power/-/commits/eas/next/integration-20210129 
> <https://gitlab.arm.com/linux-arm/linux-power/-/commits/eas/next/integration-20210129>
> > Adding a check before subtracting the values, and bailing out in such
> > case would avoid this, such as at:
> > https://gitlab.arm.com/linux-arm/linux-pg/-/commits/feec_bail_out/ 
> <https://gitlab.arm.com/linux-arm/linux-pg/-/commits/feec_bail_out/>
> >
> In your patch, you bail out the case by "go to fail", that means you
> don't use eas in such
> case. However, in the actual scene, the case often occurr when select
> cpu for small task.
> As a result, the small task would not select cpu according to the eas,
> it may affect
> power consumption?
With this patch (bailing out), the percentage of feec() returning due to 
a negative delta I get are:
on a Juno-r2, with 2 big CPUs and 4 CPUs (capacity of 383), with a 
workload running during 5s with task having a period of 16 ms and:
  - 50 tasks at 1%:   0.14%
  - 30 tasks at 1%:   0.54%
  - 10 tasks at 1%: < 0.1%
  - 30 tasks at 5%: < 0.1%
  - 10 tasks at 5%: < 0.1%
It doesn't happen so often to me.If we bail out of feec(), the task will 
still have another opportunity in the next call. However I agree this 
can lead to a bad placement when this happens.
>
> > I think a similar modification should be done in your patch. Even though
> > this is a good idea to group the calls to compute_energy() to reduce the
> > chances of having updates of utilization values in between the
> > compute_energy() calls,
> > there is still a chance to have updates. I think it happened when I
> > applied your patch.
> >
> > About changing the delta(s) from 'unsigned long' to 'long', I am not
> > sure of the meaning of having a negative delta. I thing it would be
> > better to check and fail before it happens instead.
> >
> > Regards
> >