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

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Date:	Tue, 20 Dec 2011 19:27:54 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Vince Weaver <vweaver1@...s.utk.edu>
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)


* Vince Weaver <vweaver1@...s.utk.edu> wrote:

> 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.

That does not really contradict what i said.

> 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?

At most an LLWPCB instruction is needed.

> 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.

Only if you are comparing apples to oranges: if you compare a 
full kernel based read of self-profiling counters with an RDPMC 
instruction.

But as we told you previously, you could use RDPMC under perf as 
well, last i checked PeterZ posted experimental patches for 
that. Peter, what's the status of that?

Thanks,

	Ingo
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