Re: [RFC] Energy/power monitoring within the kernel

[Guenter Roeck]

On Wed, Oct 24, 2012 at 05:37:27PM +0100, Pawel Moll wrote:
> On Tue, 2012-10-23 at 23:02 +0100, Guenter Roeck wrote:
> > > Traditionally such data should be exposed to the user via hwmon sysfs
> > > interface, and that's exactly what I did for "my" platform - I have
> > > a /sys/class/hwmon/hwmon*/device/energy*_input and this was good
> > > enough to draw pretty graphs in userspace. Everyone was happy...
> > > 
> > Only driver supporting "energy" output so far is ibmaem, and the reported energy
> > is supposed to be cumulative, as in energy = power * time. Do you mean power,
> > possibly ?
> 
> So the vexpress would be the second one, than :-) as the energy
> "monitor" actually on the latest tiles reports 64-bit value of
> microJoules consumed (or produced) since the power-up.
> 
> Some of the older boards were able to report instant power, but this
> metrics is less useful in our case.
> 
> > > Now I am getting new requests to do more with this data. In particular
> > > I'm asked how to add such information to ftrace/perf output. The second
> > > most frequent request is about providing it to a "energy aware"
> > > cpufreq governor.
> > 
> > Anything energy related would have to be along the line of "do something after a
> > certain amount of work has been performed", which at least at the surface does
> > not make much sense to me, unless you mean something along the line of a
> > process scheduler which schedules a process not based on time slices but based
> > on energy consumed, ie if you want to define a time slice not in milli-seconds
> > but in Joule.
> 
> Actually there is some research being done in this direction, but it's
> way too early to draw any conclusions...
> 
> > If so, I would argue that a similar behavior could be achieved by varying the
> > duration of time slices with the current CPU speed, or simply by using cycle
> > count instead of time as time slice parameter. Not that I am sure if such an
> > approach would really be of interest for anyone. 
> > 
> > Or do you really mean power, not energy, such as in "reduce CPU speed if its
> > power consumption is above X Watt" ?
> 
> Uh. To be completely honest I must answer: I'm not sure how the "energy
> aware" cpufreq governor is supposed to work. I have been simply asked to
> provide the data in some standard way, if possible.
> 
> > I am not sure how this would be expected to work. hwmon is, by its very nature,
> > a passive subsystem: It doesn't do anything unless data is explicitly requested
> > from it. It does not update an attribute unless that attribute is read.
> > That does not seem to fit well with the idea of tracing - which assumes
> > that some activity is happening, ultimately, all by itself, presumably
> > periodically. The idea to have a user space application read hwmon data only
> > for it to trigger trace events does not seem to be very compelling to me.
> 
> What I had in mind was similar to what adt7470 driver does. The driver
> would automatically access the device every now and then to update it's
> internal state and generate the trace event on the way. This
> auto-refresh "feature" is particularly appealing for me, as on some of
> "my" platforms can take up to 500 microseconds to actually get the data.
> So doing this in background (and providing users with the last known
> value in the meantime) seems attractive.
> 
A bad example doesn't mean it should be used elsewhere.

adt7470 needs up to two seconds for a temperature measurement cycle, and it
can not perform automatic cycles all by itself. In this context, executing
temperature measurement cycles in the background makes a lot of sense,
especially since one does not want to wait for two seconds when reading
a sysfs attribute.

But that only means that the chip is most likely not a good choice when selecting
a temperature sensor, not that the code necessary to get it working should be used
as an example for other drivers. 

Guenter

> > An exception is if a monitoring device suppports interrupts, and if its driver
> > actually implements those interrupts. This is, however, not the case for most of
> > the current drivers (if any), mostly because interrupt support for hardware
> > monitoring devices is very platform dependent and thus difficult to implement.
> 
> Interestingly enough the newest version of our platform control micro
> (doing the energy monitoring as well) can generate and interrupt when a
> transaction is finished, so I was planning to periodically update the
> all sort of values. And again, generating a trace event on this
> opportunity would be trivial.
> 
> > > Of course a particular driver could register its own perf PMU on its
> > > own. It's certainly an option, just very suboptimal in my opinion.
> > > Or maybe not? Maybe the task is so specialized that it makes sense?
> > > 
> > We had a couple of attempts to provide an in-kernel API. Unfortunately,
> > the result was, at least so far, more complexity on the driver side.
> > So the difficulty is really to define an API which is really simple, and does
> > not just complicate driver development for a (presumably) rare use case.
> 
> Yes, I appreciate this. That's why this option is actually my least
> favourite. Anyway, what I was thinking about was just a thin shin that
> *can* be used by a driver to register some particular value with the
> core (so it can be enumerated and accessed by in-kernel clients) and the
> core could (or not) create a sysfs attribute for this value on behalf of
> the driver. Seems lightweight enough, unless previous experience
> suggests otherwise?
> 
> Cheers!
> 
> PaweÅ
> 
> 
> 
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