linux-kernel - Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

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Message-ID: <alpine.DEB.2.00.1112201040190.11369@cl320.eecs.utk.edu>
Date:	Tue, 20 Dec 2011 10:48:26 -0500
From:	Vince Weaver <vweaver1@...s.utk.edu>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Avi Kivity <avi@...hat.com>,
	Robert Richter <robert.richter@....com>,
	Benjamin Block <bebl@...eta.org>,
	Hans Rosenfeld <hans.rosenfeld@....com>, <hpa@...or.com>,
	<tglx@...utronix.de>, <suresh.b.siddha@...el.com>,
	<eranian@...gle.com>, <brgerst@...il.com>,
	<Andreas.Herrmann3@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>,
	Benjamin Block <benjamin.block@....com>
Subject: Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

On Tue, 20 Dec 2011, Ingo Molnar wrote:
> Granted, LWP was mis-designed to quite a degree, those AMD chip 
> engineers should have talked to people who understand how modern 
> PMU abstractions are added to the OS kernel properly. 

You do realize that LWP was probably in design 5+ years ago, at a time 
when most Linux kernel developers wanted nothing to do with perf counters, 
and thus anyone they did contact for help would have been from the 
since-rejected perfctr or perfmon2 camp.

Also, I'm sure Linux isn't the only Operating System that they had in mind 
when designing this functionality.


Running LWP through the kernel is a foolish idea.  Does anyone have any 
numbers on what that would do to overhead?

perf_events creates huge overhead when doing self monitoring.  For simple 
self-monintoring counter reads it is an *order of magnitude* worse than
doing the same thing with perfctr.
  (see numbers here if you don't believe me:
    http://web.eecs.utk.edu/~vweaver1/projects/perf-events/benchmarks/rdtsc_overhead/ )

Vince

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