[Bugme-new] [Bug 17291] New: intel_idle causes erratic coretemp readings

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=17291

           Summary: intel_idle causes erratic coretemp readings
           Product: Drivers
           Version: 2.5
    Kernel Version: next-20100827
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Hardware Monitoring
        AssignedTo: khali at linux-fr.org
        ReportedBy: chewi at aura-online.co.uk
                CC: lenb at kernel.org
        Regression: No


I have an i7-875K and with intel_idle enabled, the readings from coretemp are
very erratic. The reading from all four cores will frequently drop very low,
sometimes even as low as 21°C, before returning to what I suppose is normal at
around 30-40°C. Usually they will all drop to the same temperature but not
always. For example, I might get 25/25/25/26. I would expect the temperature to
drop when in a high C-State but given that 21°C is more or less room
temperature and the way in which the readings suddenly rise and fall by as much
as 10°C, I think that this is not working properly. My very uneducated guess
would be that when a core is in C3 and/or C6, it cannot get a reading from the
sensor and so it gets 0 instead.

I did a bisect and found that the problem appeared as soon as intel_idle was
introduced. I can confirm that the problem is still present as of
linux-next-20100827.

I'm CC'ing Len Brown as he is the author of intel_idle.

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