(can you, perchance, look at a MUA that isn't 'broken' ?)
On Tue, Jun 11, 2019 at 09:04:30AM -0700, Andy Lutomirski wrote: > > > > On Jun 11, 2019, at 1:54 AM, Peter Zijlstra <[email protected]> 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. I agree that pm-qos is pretty terrible; but that doesn't mean we should just add random control files all over the place.
