linux-kernel - Re: [RFC] x86, perf: Add an aperfmperf driver

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Date:	Tue, 28 Apr 2015 15:43:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Brown, Len" <len.brown@...el.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [RFC] x86, perf: Add an aperfmperf driver

On Tue, Apr 28, 2015 at 3:29 PM, Brown, Len <len.brown@...el.com> wrote:
>> I think that turbostat could do some of its work without being
>> root if we had a driver like this.
>
> Note that turbostat can be run as non-root this way:
>
> # setcap cap_sys_rawio=ep ./turbostat
> # chmod +r /dev/cpu/*/msr
>
> For the debug case, there are a number of MSRs that turbostat must access,
> so would still need permission for that case (which is the only case I use:-)
>

True.  This would only get the average turbo ratio.  Of course, I
think that can be done using cpu-cycles as well.

>> Thoughts?  Would it make sense at all?  Did I wire it up right?  This is
>> the only PMU driver I've ever written, and it could have any number of
>> issues.
>
> APERF/MPERF, as with all per-thread MSRs, must be accessed
> from the local processor.  I didn't see where this driver
> distinguishes the CPU.  Also, I assume the intent is to return
> a snapshot, rather than sampling, yes?

I think that the perf core takes care of that for us, but I'm not entirely sure.

--Andy
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