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

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Message-ID: <20071119224845.GA27766@frankl.hpl.hp.com>
Date:	Mon, 19 Nov 2007 14:48:46 -0800
From:	Stephane Eranian <eranian@....hp.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	David Miller <davem@...emloft.net>, hch@...radead.org,
	akpm@...ux-foundation.org, gregkh@...e.de, mucci@...utk.edu,
	wcohen@...hat.com, robert.richter@....com,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	Stephane Eranian <eranian@....hp.com>
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

Paul,

On Tue, Nov 20, 2007 at 08:43:32AM +1100, Paul Mackerras wrote:
> David Miller writes:
> 
> > As a result I've found that perfmon2 is quite nice and allows
> > incredibly useful and powerful tools to be written.  The syscalls
> > aren't that bad and really I see not reason to block it's inclusion.
> > 
> > I rescind all of my earlier objections, let's merge this soon :-)
> 
> Strongly agree.  However, I think we need to add structure size
> arguments to most of the syscalls so we can extend them later.
> 
Yes, that is one way. It works well if you only extend structures at the end.
Given that you need to obtain the file descriptor first via a pfm_create_context
call, an alternative could be that you pass a version number to that call to
identify the version the application is requesting.

> Also, something I've been meaning to mention to Stephane is that the
> use of the cast_ulp() macro in perfmon is bogus and won't work on
> 32-bit big-endian platforms such as ppc32 and sparc32.  On such

I don't like those cast_ulp() macros. They were put there to avoid compiler
warnings on some architectures. Clearly with the big-endian issue, we need
to find something else. The bitmap*() macros make unsigned long *.

The interface uses fixed size type to ensure ABI compatibility between
32 and 64 bit modes. This way there is no need to marhsall syscall arguments
for a 32-bit app running on a 64-bit host.

Looks like we will have to use bytes (u8) instead.  This may have some
performance impact as well. Several bitmaps are used in the context/interrupt
routines. Even with u8, there is still a problem with the bitmap*() macros.
Now, only a small subset of the bitmap() macros are used, so it may be okay
to duplicate them for u8.

What do you think?

> platforms you can't take a pointer to an array of u64, cast it to
> unsigned long * and expect the kernel bitmap operations to work
> correctly on it.  At the least you also need to XOR the bit numbers
> with 32 on those platforms.  Another alternative is to define the
> bitmaps as arrays of bytes instead, which eliminates all byte ordering
> and wordsize problems (but makes it more tricky to use the kernel
> bitmap functions directly).
> 

-- 

-Stephane
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