Messages in this thread | | | Date | Mon, 13 Feb 2006 10:46:06 +1100 | From | David Gibson <> | Subject | Re: [Perfctr-devel] Re: [perfmon] perfmon2 code review: 32-bit ABI on 64-bit OS |
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On Sat, Feb 11, 2006 at 02:33:54PM -0800, Stephane Eranian wrote: > 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.
Hmm.. does the sampling buffer collect on userspace PC values, or kernel ones as well?
-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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