Re: [PATCH 1/3 v7] hwmon: Add amd_energy driver to report energy counters

[Naveen Krishna Ch <naveenkrishna.ch@gmail.com>]

Hi Alexander

On Wed, 27 May 2020 at 19:38, Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Wed, 27 May 2020, Guenter Roeck wrote:
>
> > >> I understand that. If you anticipate that the module may be inserted after a
> > >> wraparound, the driver should populate 'prev_value' with actual counter
> > >> values instead of zeros. That way the driver will properly accumulate
> > >> energy over time it's been inserted. As implemented, the driver counts
> > >> energy since boot time, minus unknown amount lost to wraparounds if the
> > >> driver was loaded too late.
> > > No problem if this module is built into the kernel.
> > > If this module is inserted at later point, unless the user keeps the
> > > counters since
> > > the boot and provide it as an input during the module insert (we can
> > > implement this).
> >
> > I won't accept such a parameter. If you may recall, I did bring this up as
> > reason why I abandoned the matching change for the coretemp driver, predicting
> > that people would complain about it. Now they do. Not surprising. We (and you)
> > will have to live with it.
Yes agree, letting user provide initial values is not appropriate. My bad.

>
> I'm not exactly complaining, I'm proposing a solution: at probe time, populate
> prev_value members with MSR values instead of zeros. That way, the module will
> correctly count energy over the time it's been loaded. It can be unloaded and
> reloaded freely, and doing so would allow to easily measure energy across large
> spans of time, which sounds like an improvement.

In the current driver, the accumulation thread starts during the probe and the
prev_value of sockets and the core0 is updated with the current MSR value.
It takes (nr_cpus - 1) iterations of the thread to populate the prev_value for
the other cores.

By populating prev_value of all the cores during probe, we can
certainly save some
information (values with in one wrap-around at the best). If this
information is
useful, i can submit a patch to do so. Guenter, could you suggest us on this.

>
> I can try to cook a corresponding patch if that sounds alright.
>
> Cheers.
> Alexander



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