Hello, On 03/04/2016 12:54 AM, Will Deacon wrote: > Hi Marc, > > On Thu, Mar 03, 2016 at 11:27:11PM +0100, Marc Kleine-Budde wrote: >> I'm using btrfs on am ARMv7 and it turns out, that the kernel has to >> fixup a lot of kernel originated alignment issues. >> >> See /proc/cpu/alignment (~4h of uptime): >>> System: 22304815 (btrfs_get_token_64+0x13c/0x148 [btrfs]) >> >> For example, when compiling the kernel on a btrfs volume the counter >> increases by 100...1000 per second. >> >> The function shown "btrfs_get_token_64()" is defined here: >>> http://lxr.free-electrons.com/source/fs/btrfs/struct-funcs.c#L53 >> ...it already uses get_unaligned_leXX accessors. >> >> Quoting a comment in arch/arm/mm/alignment.c: >> >> * ARMv6 and later CPUs can perform unaligned accesses for >> * most single load and store instructions up to word size. >> * LDM, STM, LDRD and STRD still need to be handled. >> >> But on a 32bit ARMv7 64bits are not word-sized. >> >> Is the exception and fixup overhead neglectable? Do we have to introduce >> something like HAVE_EFFICIENT_UNALIGNED_64BIT_ACCESS? > > Ouch, that trap/emulate is certainly going to have an effect on your > performance. I doubt that HAVE_EFFICIENT_UNALIGNED_ACCESS applies to > types bigger than the native word size on many architectures, so my > hunch is that the btrfs code should be checking BITS_PER_LONG or similar > to establish whether or not to break the access up into word accesses.
I've added the btrfs maintainers on Cc. > A cursory look at the network layer indicates that kind of trick is done > over there. I stumbled over this, too. Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
signature.asc
Description: OpenPGP digital signature
