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    Subject[REGRESSION] 774ac8b7eff6 ("Thermal: initialize thermal zone device correctly") causes performance drop
    Date
    Hi,

    Fedora received a bug report (https://bugzilla.redhat.com/show_bug.cgi?id=1317190)
    of a major performance drop on various bench marks and general system
    sluggishness with the 4.4.4 kernel update. The benchmarks were showing
    a reduction to about 18% performance (not minor).

    Bisection showed the first bad commit was

    commit 774ac8b7eff69e0786970157de2157e68b22f456
    Author: Zhang Rui <rui.zhang@intel.com>
    Date: Fri Oct 30 16:31:47 2015 +0800

    Thermal: initialize thermal zone device correctly

    commit bb431ba26c5cd0a17c941ca6c3a195a3a6d5d461 upstream.

    After thermal zone device registered, as we have not read any
    temperature before, thus tz->temperature should not be 0,
    which actually means 0C, and thermal trend is not available.
    In this case, we need specially handling for the first
    thermal_zone_device_update().

    Both thermal core framework and step_wise governor is
    enhanced to handle this. And since the step_wise governor
    is the only one that uses trends, so it's the only thermal
    governor that needs to be updated.

    Tested-by: Manuel Krause <manuelkrause@netscape.net>
    Tested-by: szegad <szegadlo@poczta.onet.pl>
    Tested-by: prash <prash.n.rao@gmail.com>
    Tested-by: amish <ammdispose-arch@yahoo.com>
    Tested-by: Matthias <morpheusxyz123@yahoo.de>
    Reviewed-by: Javi Merino <javi.merino@arm.com>
    Signed-off-by: Zhang Rui <rui.zhang@intel.com>
    Signed-off-by: Chen Yu <yu.c.chen@intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>



    Reverting this plus to other commits in the series (a67208e94d94
    "Thermal: handle thermal zone device properly during system sleep"
    and 27f356149d59 "Thermal: do thermal zone update after a cooling
    device registered") confirmed the performance was back to normal.

    Bugzilla has the full discussion but this comment from one of the
    reporters sums it up:

    "In 4.4.3 and prior, my 2.40 MHz processor would fluctuate between
    1000 and 3400 MHz. In 4.4.4, the processor would fluctuate between
    400 and 700 MHz, according to /proc/cpuinfo.

    Setting /sys/devices/system/cpu/cpufreq/policy0/scaling_governor to
    performance, instead of the default "powersave" forces the CPU to
    2400 MHz, and improves performance greatly, but still not to the
    same level as in 4.4.3."

    Any ideas?

    Thanks,
    Laura

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