[Bug 2054391] Re: Fix CPU thermal sensors enumeration

[Robie Basak]

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** Changed in: thermald (Ubuntu Jammy)
       Status: Confirmed => Fix Committed

** Tags added: verification-needed verification-needed-jammy

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https://bugs.launchpad.net/bugs/2054391

Title:
  Fix  CPU thermal sensors enumeration

Status in HWE Next:
  New
Status in thermald package in Ubuntu:
  Fix Released
Status in thermald source package in Jammy:
  Fix Committed

Bug description:
  [Impact]
  Some CPU sensors are not enumerated, this can make thermald deviates from the correct behavior of the CPU TDP.

  [Fix]
  Traverse all sensors under hwmon sysfs directory to make sure everything is enumerated.

  [Test]
  Check the output of thermald. Once the fix is in place, thermal zones that are previously omitted now shows up:
  [INFO]Zone 1: AMBF, Active:1 Bind:1 Sensor_cnt:1
  To do so
  0. get a large machine which will have more thermal zones
  1. stop the potentially auto-running service
     $ systemctl stop thermald
  2. run the daemon in foreground with loglevel to see what is going on.
     On many modern systemd (=the large ones) it might not know the CPUid,
     to bypass that for the test you can ask it to ignore the check
     $ sudo thermald --no-daemon --loglevel=info --ignore-cpuid-check
  3. check the output
     On init the system will be probed and that will show something like:

  ...
   ZONE DUMP BEGIN
  [1718954645][INFO]Zone 2: cpu, Active:1 Bind:0 Sensor_cnt:1
  ...
  [1718954645][INFO]Zone 3: cpu, Active:1 Bind:0 Sensor_cnt:1
  ...
   ZONE DUMP END

  In here, on systems with many thermal zones one would before the fix
  only see a few, and with the fix more zones.

  [Where problems could occur]
  Since the new logic traverse the whole hwmon sysfs, the startup time can take slightly longer.

  [racb] Existing users' systems may have bad or otherwise irrelevant or
  out of scope sensors that may not have been causing misbehaviour due
  to being skipped, but after the fix, they would face a regression. I'm
  not sure that we can realistically identify such cases though, and it
  seems reasonable to favour correct systems over misbehaving ones.

  [racb] Similar to my previous point, we may pick up additional sensor
  data that we shouldn't do due to inadequate filtering, causing
  incorrect behaviour but this time it would be a bug in our filtering
  rather than misbehaving existing systems. In mitigation, I see that
  the fixed version has been released in Kinetic, so has had some real
  world testing, and I see no indication upstream or in Launchpad that
  this was a problem in practice.

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