On Mon, Jun 17, 2019 at 08:14:44AM -0700, Andy Lutomirski wrote: > On Tue, Jun 11, 2019 at 10:27 AM Peter Zijlstra <pet...@infradead.org> wrote: > > > > > > (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 <pet...@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. > > > > I agree that pm-qos is pretty terrible; but that doesn't mean we should > > just add random control files all over the place. > > I don't think pm-qos is expressive enough. It seems entirely > reasonable to want to do a C0.1 wait for lower latency *while waiting* > but still want full power-saving idle when not waiting. > > Do we even know what the C0.2 and C0.1 latencies are? And why is this > thing an MSR instead of a flag passed to UMWAIT?
I will still keep this sysfs interface in the next version of patches, right? Thanks. -Fenghua
