Messages in this thread | | | Subject | Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state | From | Andy Lutomirski <> | Date | Tue, 11 Jun 2019 09:04:30 -0700 |
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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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