Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190611085410.GT3436@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Jun 2019 09:04:30 -0700
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Content-Transfer-Encoding: quoted-printable
Message-Id: <0D67CEAC-9710-4ECB-9248-75B48542FF82@amacapital.net>
References: <1559944837-149589-1-git-send-email-fenghua.yu@intel.com> <1559944837-149589-4-git-send-email-fenghua.yu@intel.com> <20190611085410.GT3436@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 11, 2019, at 1:54 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>> 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.
>>=20
>> By default, C0.2 is enabled and the user wait instructions result in
>> lower power consumption with slower wakeup time.
>>=20
>> 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.
>>=20
>> 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.=

>=20
> We already have an interface for applications to convey their latency
> requirements (pm-qos). We do not need another magic sys variable.

I=E2=80=99m not sure I agree.  This isn=E2=80=99t an overall latency request=
, and setting an absurdly low pm_qos will badly hurt idle power and turbo pe=
rformance.  Also, pm_qos isn=E2=80=99t 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 b=
ox, I directly disable c states instead of messing with pm_qos.)

I do wonder whether anyone will ever use this particular control, though.=