From: Arnd Bergmann <arnd@kernel.org>
To: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: linux-arch <linux-arch@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Vineet Gupta <vgupta@synopsys.com>,
Yoshinori Sato <ysato@users.sourceforge.jp>,
Rich Felker <dalias@libc.org>,
Linux-sh list <linux-sh@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 03/13] sh: remove unaligned access for sh4a
Date: Sat, 15 May 2021 22:10:23 +0200 [thread overview]
Message-ID: <CAK8P3a1VOgW+oT9cGGYyLEt4jr+zrZyA6fz66AdSZuSgoq5xaQ@mail.gmail.com> (raw)
In-Reply-To: <71b5d15d-7bd2-aa08-cc0a-3caccf9c66c8@physik.fu-berlin.de>
On Sat, May 15, 2021 at 5:36 PM John Paul Adrian Glaubitz
<glaubitz@physik.fu-berlin.de> wrote:
> On 5/14/21 2:22 PM, Arnd Bergmann wrote:
> >> My Renesas SH4-Boards actually run an sh4a-Kernel, not an sh4-Kernel:
> >>
> >> root@tirpitz:~> uname -a
> >> Linux tirpitz 5.11.0-rc4-00012-g10c03c5bf422 #161 PREEMPT Mon Jan 18 21:10:17 CET 2021 sh4a GNU/Linux
> >> root@tirpitz:~>
> >>
> >> So, if this change reduces performance on sh4a, I would rather not merge it.
> >
> > It only makes a difference in very specific scenarios in which unaligned
> > accesses are done in a fast path, e.g. when forwarding network packet
> > at a high rate on a big-endian kernel (little-endian kernels wouldn't run into
> > this on IP headers). If you have a use case for this machine on which the
> > you can show a performance regression, I can add a patch on top to put
> > the optimized sh4a get_unaligned_le32() back. Dropping this patch
> > altogether would make the series much more complex because most of
> > the associated code gets removed in the end.
>
> Hmm, okay. But why does code which sits below arch/sh have to be removed anyway?
>
> I don't fully understand why it poses any maintenance burden/
What I'm removing is the part that lets architectures override the
generic version.
> > As I mentioned, supporting "movua" in the compiler likely has a much
> > larger impact on performance, as it would also help in user space, and
> > it should improve the networking case on little-endian kernels by replacing
> > the four separate byte loads/shift pairs with a movua plus a byteswap.
>
> The problem is that - at least in Debian - we use the sh4 baseline while the kernel
> supports both sh4 and sh4a, so we can't use any of these instructions in userland at
> the moment.
I tried building an sh7785lcr_defconfig with and without the patch,
and found that
the only affected files are:
- in-kernel nfs client
- crc32c/sha1/sha256 hash functions
- device probing for libata, scsi-core, scsi-disk, hid, r8168
(should not matter after boot)
- msdos partition parsing
Any nfs client performance difference is probably not even measurable even
at gigabit ethernet speed.
I see that the hash functions are notably different, but I don't know if the
output from the new generic code is actually better or worse than the
original. If you do think this is important, please try the version from
https://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic.git
unaligned-sh4a
against the version without the last change in that series. If you can find
a relevant test case that exercises it, you may want to add a custom
implementation of the hash functions as well.
Arnd
next prev parent reply other threads:[~2021-05-15 20:11 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-14 10:00 [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 01/13] asm-generic: use asm-generic/unaligned.h for most architectures Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 02/13] openrisc: always use unaligned-struct header Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 03/13] sh: remove unaligned access for sh4a Arnd Bergmann
2021-05-14 10:34 ` John Paul Adrian Glaubitz
2021-05-14 12:22 ` Arnd Bergmann
2021-05-15 15:36 ` John Paul Adrian Glaubitz
2021-05-15 20:10 ` Arnd Bergmann [this message]
2021-05-14 10:00 ` [PATCH v2 04/13] m68k: select CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 05/13] powerpc: use linux/unaligned/le_struct.h on LE power7 Arnd Bergmann
2021-05-14 11:48 ` Segher Boessenkool
2021-05-14 13:02 ` Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 06/13] asm-generic: unaligned: remove byteshift helpers Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 07/13] asm-generic: unaligned always use struct helpers Arnd Bergmann
2021-05-17 21:53 ` Eric Biggers
2021-05-18 7:25 ` Arnd Bergmann
2021-05-18 14:56 ` Linus Torvalds
2021-05-18 15:41 ` Arnd Bergmann
2021-05-18 16:12 ` Linus Torvalds
2021-05-18 18:09 ` Jason A. Donenfeld
2021-05-18 20:51 ` Arnd Bergmann
2021-05-18 21:31 ` Eric Biggers
2021-05-18 21:14 ` David Laight
2021-05-14 10:00 ` [PATCH v2 08/13] partitions: msdos: fix one-byte get_unaligned() Arnd Bergmann
2021-05-17 10:28 ` Christoph Hellwig
2021-05-17 10:44 ` Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 09/13] apparmor: use get_unaligned() only for multi-byte words Arnd Bergmann
2021-05-14 10:00 ` [PATCH v2 10/13] mwifiex: re-fix for unaligned accesses Arnd Bergmann
2021-05-15 6:22 ` Kalle Valo
2021-05-15 9:01 ` Arnd Bergmann
2021-05-15 18:23 ` Kalle Valo
2021-05-14 10:00 ` [PATCH v2 11/13] netpoll: avoid put_unaligned() on single character Arnd Bergmann
2021-05-14 10:01 ` [PATCH v2 12/13] asm-generic: uaccess: 1-byte access is always aligned Arnd Bergmann
2021-05-15 18:41 ` Randy Dunlap
2021-05-15 20:16 ` Arnd Bergmann
2021-05-14 10:01 ` [PATCH v2 13/13] asm-generic: simplify asm/unaligned.h Arnd Bergmann
2021-05-14 10:35 ` David Laight
2021-05-14 17:32 ` [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Linus Torvalds
2021-05-14 18:51 ` Vineet Gupta
2021-05-14 19:22 ` Linus Torvalds
2021-05-14 19:45 ` Vineet Gupta
2021-05-14 20:19 ` Linus Torvalds
2021-05-14 19:31 ` Arnd Bergmann
2021-12-16 17:29 ` Ard Biesheuvel
2021-12-16 17:42 ` Linus Torvalds
2021-12-16 17:49 ` David Laight
2021-12-16 18:56 ` Segher Boessenkool
2021-12-17 12:34 ` David Laight
2021-12-17 13:35 ` Segher Boessenkool
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