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

[Paul Mackerras <paulus@samba.org>]

David Miller writes:

> From: Paul Mackerras <paulus@samba.org>
> Date: Thu, 15 Nov 2007 10:12:22 +1100
> 
> > *I* never had a problem with a few extra system calls.  I don't
> >  understand why you (apparently) do.
> 
> We're stuck with them forever, they are hard to version and extend
> cleanly.
> 
> Those are my main objections.

The first is valid (for suitable values of "forever") but applies to
any user/kernel interface, not just system calls.

As for the second (hard to version) I don't see why it applies to
syscalls specifically more than to other interfaces.  It's just a
matter of designing it correctly in the first place.  For example, the
sys_swapcontext system call we have on powerpc takes an argument which
is the size of the ucontext_t that userland is using, which allows us
to extend it in future if necessary.  (Note that I'm not saying that
the current perfmon2 interfaces are well-designed in this respect.)

The third (hard to extend cleanly) is a good point, and is a valid
criticism of the current set of perfmon2 system calls, I think.
However, the goal of being able to extend the interface tends to be in
opposition to the goal of having strong typing of the interface.
Things like a multiplexed syscall or an ioctl are much easier to
extend but that is at the expense of losing strong typing.  Something
like my transaction() (or your weird kind of read() :) also provides
extensibility but loses type safety to some degree.

Also, as Andi says, this is core CPU state that we are dealing with,
not some I/O device, so treating the whole of perfmon2 (or any
performance monitoring infrastructure) as a driver doesn't fit very
well, and in fact system calls are appropriate.  Just like we don't
try to make access to debugging facilities fit into a driver, we
shouldn't make performance monitoring fit into a driver either.

Paul.
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