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

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Message-Id: <200711141125.18568.nickpiggin@yahoo.com.au>
Date:	Wed, 14 Nov 2007 11:25:17 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Miller <davem@...emloft.net>
Cc:	paulus@...ba.org, hch@...radead.org, akpm@...ux-foundation.org,
	gregkh@...e.de, mucci@...utk.edu, eranian@....hp.com,
	wcohen@...hat.com, robert.richter@....com,
	linux-kernel@...r.kernel.org, andi@...stfloor.org
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

On Wednesday 14 November 2007 22:58, David Miller wrote:
> From: Nick Piggin <nickpiggin@...oo.com.au>
> Date: Wed, 14 Nov 2007 10:49:48 +1100
>
> > On Wednesday 14 November 2007 22:44, Paul Mackerras wrote:
> > > David Miller writes:
> > > > This is my impression too, all of the things being done with
> > > > a slew of system calls would be better served by real special
> > > > files and appropriate fops.
> > >
> > > Special files and fops really only work well if you can coerce the
> > > interface into one where data flows predominantly one way.  I don't
> > > think they work so well for something that is more like an RPC across
> > > the user/kernel barrier.  For that a system call is better.
> > >
> > > For instance, if you have something that kind-of looks like
> > >
> > > 	read_pmds(int n, int *pmd_numbers, u64 *pmd_values);
> > >
> > > where the caller supplies an array of PMD numbers and the function
> > > returns their values (and you want that reading to be done atomically
> > > in some sense), how would you do that using special files and fops?
> >
> > Could you implement it with readv()?
>
> Sure, why not?  Just cook up an iovec.  pmd_numbers goes to offset
> X and pmd_values goes to offset Y, with some helpers like what
> we have in the networking already for recvmsg.
>
> But why would you want readv() for this?  The syscall thing
> Paul asked me to translate into a read() doesn't provide
> iovec-like behavior so I don't see why readv() is necessary
> at all.

Ah sorry, that's what I get for typing before I think: of course
readv doesn't vectorise the right part of the equation.

What I really mean is a readv-like syscall, but one that also
vectorises the file offset. Maybe this is useful enough as a generic
syscall that also helps Paul's example...

Of course, I guess this all depends on whether the atomicity is an
important requirement. If not, you can obviously just do it with
multiple read syscalls...
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