linux-kernel - Re: [PATCH 1/2] Modify cpupower to schedule itself on cores it is reading MSRs from

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Date:   Mon, 7 Oct 2019 21:11:30 +0000
From:   "Natarajan, Janakarajan" <Janakarajan.Natarajan@....com>
To:     Thomas Renninger <trenn@...e.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Pu Wen <puwen@...on.com>, Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Allison Randal <allison@...utok.net>,
        Richard Fontana <rfontana@...hat.com>,
        Thomas Renninger <trenn@...e.com>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 1/2] Modify cpupower to schedule itself on cores it is
 reading MSRs from

On 10/5/2019 7:40 AM, Thomas Renninger wrote:
> Hi,
>
> On Wednesday, October 2, 2019 4:45:03 PM CEST Natarajan, Janakarajan wrote:
>> On 9/27/19 4:48 PM, Thomas Renninger wrote:
>>
>>> On Friday, September 27, 2019 6:07:56 PM CEST  Natarajan, Janakarajan
>>> wrote:
>>>> On 9/18/2019 11:34 AM, Natarajan, Janakarajan wrote:
>   
>> On a 256 logical-cpu Rome system we see C0 value from cpupower output go
>> from 0.01 to ~(0.1 to 1.00)
>>
>> for all cpus with the 1st patch.
>>
>> However, this goes down to ~0.01 when we use the RDPRU instruction
>> (which can be used to get
>>
>> APERF/MPERF from CPL > 0) and avoid using the msr module (patch 2).
> And this one only exists on latest AMD cpus, right?


Yes. The RDPRU instruction exists only on AMD cpus.


>
>> However, for systems that provide an instruction  to get register values
>> from userspace, would a command-line parameter be acceptable?
> Parameter sounds like a good idea. In fact, there already is such a paramter.
> cpupower monitor --help
>         -c
>             Schedule  the  process  on every core before starting and ending
> measuring.  This could be needed for the Idle_Stats monitor when no other MSR
> based monitor (has to be run on the core that is measured) is run in parallel.
> This is to wake up the processors from deeper sleep states and let the kernel
> reaccount its cpuidle (C-state) information before reading the cpuidle timings
> from sysfs.
>
> Best is you exchange the order of your patches. The 2nd looks rather straight
> forward and you can add my reviewed-by.


The RDPRU instruction reads the APERF/MPERF of the cpu on which it is 
running. If we do

not schedule it on each cpu specifically, it will read the APERF/MPERF 
of the cpu in which it runs/might

happen to run on, which will not be the correct behavior.


>
> If you still need adjustings with -c param, they can be discussed separately.


The -c parameter causes cpupower to schedule itself on each of the cpus 
of the system in a loop.

After the loop the cpupower starts the measurement of APERF/MPERF of 
each cpu.

This doesn't offer the behavior needed to use RDPRU, which requires 
cpupower to execute on the cpu

whose APERF/MPERF values we are interested in.


Thanks,

Janak


> It would also be nice to mention in which case it makes sense to use it in the
> manpage or advantages/drawbacks if you don't.
>
> Thanks!
>
>      Thomas
>
>
>