Messages in this thread | | | Date | Sat, 11 Feb 2006 14:33:54 -0800 | From | Stephane Eranian <> | Subject | Re: [perfmon] perfmon2 code review: 32-bit ABI on 64-bit OS |
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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.
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