Re: [PATCH v2] ppc: cpufreq: disable preemption while checking CPU throttling state

Hi Michael,

On Tue, Nov 08, 2016 at 11:21:23AM +1100, Michael Ellerman wrote:

Denis Kirjanov [off-list ref] writes:

quoted

On 11/7/16, Michael Ellerman [off-list ref] wrote:

quoted

Denis Kirjanov [off-list ref] writes:

quoted

    [   67.700897] BUG: using smp_processor_id() in preemptible [00000000] code: cat/7343
    [   67.700988] caller is .powernv_cpufreq_throttle_check+0x2c/0x710

When did this break?

I think that problem exists since commit
09a972d1620934142d30cfda455ffe429af751c4 ("cpufreq: powernv: Report
cpu frequency throttling") but I can be wrong

Yep that looks right to me, thanks.

Gautham, how bad is the bug here, do we want to send this to stable?

Not so bad that it needs to be sent to stable immediately.

On CONFIG_PREEMPT enabled systems, if task executing
powernv_cpufreq_throttle_check can be switched to a CPU on a different
chip, then we will end up incorrectly attributing the throttle
statistics of the new chip to the chip where it previously ran. These
throttle statistics aren't used inside the kernel for any computation,
but are reported to the user via /sysfs for them to account for any
variance during a benchmark run.

It would be good to include this in the 4.9 fixes though. 

cheers