Sensor Value PropertiesChanged Events

[Bills, Jason M]

Hi All,

There is an issue and idea that James Feist and I chatted about to maybe 
relieve some of our D-Bus traffic.

A major contributor to our D-Bus traffic (as seen in dbus-monitor) is 
the polling sensors updating the xyz.openbmc_project.Sensor.Value.Value 
property on each polling loop, which generates a PropertiesChanged 
signal for every sensor on every polling loop (once per second?).

The concern is that more important D-Bus messages could be getting 
delayed as D-Bus processes these Sensor Value signals.

The idea to fix this is to change the sensors with a custom getter on 
the Value property, so the last read can be pulled from D-Bus using a 
get-property call, but it would no longer signal a PropertiesChanged event.

I pushed a proposed change here: 
https://gerrit.openbmc-project.xyz/c/openbmc/dbus-sensors/+/40199.

Our original assumption was that nobody was matching on this 
PropertiesChanged signal for the Value property; however, it was pointed 
out to me today, that PID control has a match for it and may be using it.

So, I wanted to start a broader community discussion about this issue:

1. Is this a real concern or are PropertiesChanged signals so 
lightweight that removing them won't help with D-Bus load?

2. Does anyone need to match on sensor Value property updates or is 
reading them with get-property enough?

3. Does PID control use the Value match?  If so and there are benefits 
to removing these signals, could PID control manage without them?


As a side note, I still have two remaining services that publish 
PropertiesChanged events on sensor Value properties:

PWM Sensors.  I have a proposed (and untested) change here: 
gerrit.openbmc-project.xyz/c/openbmc/dbus-sensors/+/40200.

A Power sensor, that I will track down based on this discussion.

Thanks!
-Jason