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

[Peter Zijlstra]

(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 <peterz@xxxxxxxxxxxxx> 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.
> 
> Iâm not sure I agree.  This isnât an overall latency request, and
> setting an absurdly low pm_qos will badly hurt idle power and turbo
> performance.  Also, pm_qos isnât 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.