linux-kernel - Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation

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Date:   Tue, 27 Dec 2016 12:00:11 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Shivappa Vikas <vikas.shivappa@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        ravi.v.shankar@...el.com, tony.luck@...el.com,
        fenghua.yu@...el.com, davidcc@...gle.com, eranian@...gle.com,
        hpa@...or.com
Subject: Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation

Shivappa Vikas <vikas.shivappa@...el.com> writes:
>
> Ok , looks like the interface  is the problem. Will try to fix
> this. We are just trying to have a light weight monitoring
> option so that its reasonable to monitor for a
> very long time (like lifetime of process etc). Mainly to not have all
> the perf scheduling overhead.

That seems like an odd reason to define a completely new user interface.
This is to avoid one MSR write for a RMID change per context switch
in/out cgroup or is it other code too?

Is there some number you can put to the overhead?
Or is there some other overhead other than the MSR write
you're concerned about?

Do you have an ftrace or better PT trace with the overhead before-after?

Perhaps some optimization could be done in the code to make it faster,
then the new interface wouldn't be needed.

FWIW there are some pending changes to context switch that will
eliminate at least one common MSR write [1]. If that was fixed
you could do the RMID MSR write "for free"

-Andi

[1] https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/fsgsbase