Re: [perfmon] perfmon2 code review: 32-bit ABI on 64-bit OS

[Stephane Eranian]

Hello,

On Sun, Feb 12, 2006 at 12:16:25AM +0600, Philip Mucci wrote:
> > 
> > On some 64-bit arches (e.g. x86_64), most userspace code is 64-bit,
> > while on others (e.g. powerpc), most is 32-bit.  Reducing the number of
> > things that a userspace tool or library writer can trip over seems like
> > a good thing here, even if it slightly complicates perfmon's internals.
> > 
> > > Note that there are similar issues with the remapped sampling buffer.
> > > There, you need to explicitly compile your tool with a special option
> > > to force certain types to be 64-bit (size_t, void *).
> > 
> > It's pretty normal to just use 64-bit quantities in these cases, and
> > cast appropriately.
> 
> I agree with Bryan. Stephane, do you have any quantitative data for how
> much more expensive going to 64 bit quantities would be? Which
> performance critical operations access this structure? AFAIK, any
> performance monitoring system call is already slow by nature...and thus
> an additional dozen cycles isn't going to make a difference. Of course,
> if this structure needs to be read/written by get_pmd, including the
> userspace version (+ mmap offset), then the extra overhead should be
> considered. 
> 
I think I can easily convert the bitmasks to be u64 on all platforms.
I don't think it will negatively impact performance on 32-bit applications.

The sampling buffer is another matter. It is directly remapped. The default
format, exposes size_t and void *. The size_t is not on the critical
path, it is used to specify the buffer size. If we expose as 64-bit,
we need to check on 32-bit system that the value is below 4GB and cast
to size_t.

The most challenging piece is the IP (program pointer) that is in every
sample. Today it is defined as unsigned long because this is fairly
natural for a code address. The 64bit OS captures addresses as 64-bit,
the 32-bit monitoring tool running on top has to consume them as 64-bit
addresses, so u64 would be fine. 

But not on a 32-bit kernel with a 32-bit tool, addresses exported as u64
would certainly work but consume double to buffer space, and that is a
more serious issue in my mind.

What we need is:
	1/ 32-bit OS: IP is 32-bit in the sampling buffer
	2/ 64-bit OS: IP is 64-bit in the sampling buffer

Because of 32-bit ABI tool running on 2/, the IP would have
to be defined as u64. But then it would be overkill on 1/.

The problem is in the user level header file for the sampling buffer.
We would need a data type that is 64-bit for IP if the host OS is 64-bit
(regardless of the ABI used by the tool, i.e., the compiler). And a data
type that is 32-bit on 32-bit OS. The problem is that there is no compiler
flag or header flag somewhere that could guide the compiler. In the case
of MIPS, we have defined a libpfm compile flags that indicates we want
the 64-bit OS definition when compiling for a 32-bit application.

-- 
-Stephane
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