Re: [perfmon] Re: [perfmon2] perfmon2 merge news

[Andi Kleen <andi@firstfloor.org>]

Christoph Hellwig <hch@infradead.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