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 10:54:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     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 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.