From: mkl@pengutronix•de (Marc Kleine-Budde)
To: linux-arm-kernel@lists•infradead.org
Subject: btrfs_get_token_64() alignment problem on ARM (was: Re: DWord alignment on ARMv7)
Date: Fri, 4 Mar 2016 09:01:44 +0100 [thread overview]
Message-ID: <56D940E8.2030202@pengutronix.de> (raw)
In-Reply-To: <20160303235426.GA11237@arm.com>
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 |
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next prev parent reply other threads:[~2016-03-04 8:01 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-03 22:27 DWord alignment on ARMv7 Marc Kleine-Budde
2016-03-03 23:54 ` Will Deacon
2016-03-04 8:01 ` Marc Kleine-Budde [this message]
2016-03-04 9:16 ` btrfs_get_token_64() alignment problem on ARM (was: Re: DWord alignment on ARMv7) David Sterba
2016-03-04 10:48 ` DWord alignment on ARMv7 Ard Biesheuvel
2016-03-04 11:02 ` Russell King - ARM Linux
2016-03-04 11:14 ` Ard Biesheuvel
2016-03-04 11:19 ` Marc Kleine-Budde
2016-03-04 11:26 ` Ard Biesheuvel
2016-03-04 11:38 ` Arnd Bergmann
2016-03-04 11:44 ` Ard Biesheuvel
2016-03-04 13:30 ` Arnd Bergmann
2016-03-04 13:33 ` Ard Biesheuvel
2016-03-04 13:46 ` Russell King - ARM Linux
2016-03-04 14:41 ` Arnd Bergmann
2016-03-04 14:56 ` Russell King - ARM Linux
2016-03-04 15:49 ` Arnd Bergmann
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