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

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Date:   Fri, 27 Sep 2019 23:48:16 +0200
From:   Thomas Renninger <trenn@...e.de>
To:     " Natarajan, Janakarajan " <Janakarajan.Natarajan@....com>
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 Friday, September 27, 2019 6:07:56 PM CEST  Natarajan, Janakarajan  wrote:
> On 9/18/2019 11:34 AM, Natarajan, Janakarajan wrote:

> > This is advantageous because an IPI is not generated when a read_msr() is
> > executed on the local logical CPU thereby reducing the chance of having
> > APERF and MPERF being out of sync.
> > +	if (sched_setaffinity(getpid(), sizeof(set), &set) == -1) {
> > +		dprint("Could not migrate to cpu: %d\n", cpu);
> > +		return 1;

On a 80 core cpu the process would be pushed around through the
system quite a lot. 
This might affect what you are measuring or the other measure values?
Otherwise it's the kernel's MSR read only, not the whole cpupower process,
right? No idea about the exact overhead, though. Others in CC list should
know.

Afaik msr reads through msr module should be avoided anyway?
Those which are worth it are abstracted through sysfs nowadays?

For aperf/mperf it might make sense to define a sysfs file where you
can read both, as this is what you always need?

It would take a while, but could be a longterm solution which is also
usable in secure boot or without msr module case.

      Thomas