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

    (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@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.

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