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SubjectRe: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state
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> On Jun 11, 2019, at 1:54 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, Jun 07, 2019 at 03:00:35PM -0700, Fenghua Yu wrote:
>> C0.2 state in umwait and tpause instructions can be enabled or disabled
>> on a processor through IA32_UMWAIT_CONTROL MSR register.
>>
>> By default, C0.2 is enabled and the user wait instructions result in
>> lower power consumption with slower wakeup time.
>>
>> But in real time systems which require faster wakeup time although power
>> savings could be smaller, the administrator needs to disable C0.2 and all
>> C0.2 requests from user applications revert to C0.1.
>>
>> A sysfs interface "/sys/devices/system/cpu/umwait_control/enable_c02" is
>> created to allow the administrator to control C0.2 state during run time.
>
> We already have an interface for applications to convey their latency
> requirements (pm-qos). We do not need another magic sys variable.

I’m not sure I agree. This isn’t an overall latency request, and setting an absurdly low pm_qos will badly hurt idle power and turbo performance. Also, pm_qos isn’t exactly beautiful.

(I speak from some experience. I may be literally the only person to write a driver that listens to dev_pm_qos latency requests. And, in my production box, I directly disable c states instead of messing with pm_qos.)

I do wonder whether anyone will ever use this particular control, though.
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