Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state

From: Peter Zijlstra
Date: Tue Jun 11 2019 - 04:58:45 EST

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.