linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <p734pfp6lyp.fsf@bingen.suse.de>
Date:	Wed, 14 Nov 2007 13:38:38 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, Philip Mucci <mucci@...utk.edu>,
	eranian@....hp.com, William Cohen <wcohen@...hat.com>,
	Robert Richter <robert.richter@....com>,
	linux-kernel@...r.kernel.org, Perfmon <perfmon@...ali.hpl.hp.com>,
	Andi Kleen <andi@...stfloor.org>,
	perfmon2-devel@...ts.sourceforge.net,
	OSPAT devel <ospat-devel@...utk.edu>,
	papi list <ptools-perfapi@...utk.edu>
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

Christoph Hellwig <hch@...radead.org> writes:
>
> I've done this a gazillion times before, so maybe instead of beeing a lazy
> bastard you could look up mailinglist archive.  It's not like this is the
> first discussion of perfmon.  But to get start look at the systems calls,
> many of them are beasts like:
>
>   int pfm_read_pmds(int fd, pfarg_pmd_t *pmds, int n)
>
> This is basically a read(2) (or for other syscalls a write) on something

At least for x86 and I suspect some 1other architectures we don't
initially need a syscall at all for this. There is an instruction
RDPMC who can read a performance counter just fine. It is also much
faster and generally preferable for the case where a process measures
events about itself. In fact it is essential for one of the use cases
I would like to see perfmon used (replacement of RDTSC for cycle
counting) 

Later a syscall might be needed with event multiplexing, but that seems
more like a far away non essential feature.

> else than the file descriptor provided to the system call.   The right thing

I don't like read/write for this too much. I think it's better to
have individual syscalls.  After all that is CPU state and having
syscalls for that does seem reasonable.

-Andi
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