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

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Date:   Tue, 11 Jun 2019 09:04:30 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state



> On Jun 11, 2019, at 1:54 AM, Peter Zijlstra <peterz@...radead.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.