From: Yury Norov <ynorov@caviumnetworks.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Miller <davem@davemloft.net>, <arnd@arndb.de>,
<linux-arm-kernel@lists.infradead.org>,
<linux-kernel@vger.kernel.org>, <linux-doc@vger.kernel.org>,
<linux-arch@vger.kernel.org>, <linux-s390@vger.kernel.org>,
<libc-alpha@sourceware.org>, <schwidefsky@de.ibm.com>,
<heiko.carstens@de.ibm.com>, <pinskia@gmail.com>,
<broonie@kernel.org>, <joseph@codesourcery.com>,
<christoph.muellner@theobroma-systems.com>,
<bamvor.zhangjian@huawei.com>, <szabolcs.nagy@arm.com>,
<klimov.linux@gmail.com>, <Nathan_Lynch@mentor.com>,
<agraf@suse.de>, <Prasun.Kapoor@caviumnetworks.com>,
<kilobyte@angband.pl>, <geert@linux-m68k.org>,
<philipp.tomsich@theobroma-systems.com>
Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation
Date: Fri, 27 May 2016 03:37:53 +0300 [thread overview]
Message-ID: <20160527003753.GA14247@yury-N73SV> (raw)
In-Reply-To: <20160526222943.GA16729@MBP.local>
On Thu, May 26, 2016 at 11:29:45PM +0100, Catalin Marinas wrote:
> On Thu, May 26, 2016 at 11:48:19PM +0300, Yury Norov wrote:
> > On Wed, May 25, 2016 at 02:28:21PM -0700, David Miller wrote:
> > > From: Arnd Bergmann <arnd@arndb.de>
> > > Date: Wed, 25 May 2016 23:01:06 +0200
> > >
> > > > On Wednesday, May 25, 2016 1:50:39 PM CEST David Miller wrote:
> > > >> From: Arnd Bergmann <arnd@arndb.de>
> > > >> Date: Wed, 25 May 2016 22:47:33 +0200
> > > >>
> > > >> > If we use the normal calling conventions, we could remove these overrides
> > > >> > along with the respective special-case handling in glibc. None of them
> > > >> > look particularly performance-sensitive, but I could be wrong there.
> > > >>
> > > >> You could set the lowest bit in the system call entry pointer to indicate
> > > >> the upper-half clears should be elided.
> > > >
> > > > Right, but that would introduce an extra conditional branch in the syscall
> > > > hotpath, and likely eliminate the gains from passing the loff_t arguments
> > > > in a single register instead of a pair.
> > >
> > > Ok, then, how much are you really gaining from avoiding a 'shift' and
> > > an 'or' to build the full 64-bit value? 3 cycles? Maybe 4?
> >
> > 4 cycles in kernel and ~same cost in glibc to create a pair.
>
> It would take a single instruction per argument in the kernel to do
> shift+or and maybe 1-2 more instructions to move the remaining arguments
> in place (we do this for a few wrappers in arch/arm64/kernel/entry32.S).
> And the glibc counterpart.
>
> > And 8 'mov's that exist for every syscall, even yield().
> >
> > > And the executing the wrappers, those have a non-trivial cost too.
> >
> > The cost is pretty trivial though. See kernel/compat_wrapper.o:
> > COMPAT_SYSCALL_WRAP2(creat, const char __user *, pathname, umode_t, mode);
> > 0: a9bf7bfd stp x29, x30, [sp,#-16]!
> > 4: 910003fd mov x29, sp
> > 8: 2a0003e0 mov w0, w0
> > c: 94000000 bl 0 <sys_creat>
> > 10: a8c17bfd ldp x29, x30, [sp],#16
> > 14: d65f03c0 ret
>
> I would say the above could be more expensive than 8 movs (16 bytes to
> write, read, a branch and a ret). You can also add the I-cache locality,
> having wrappers for each syscalls instead of a single place for zeroing
> the upper half (where no other wrapper is necessary).
>
> Can we trick the compiler into doing a tail call optimisation. This
> could have simply been:
>
> COMPAT_SYSCALL_WRAP2(creat, ...):
> mov w0, w0
> b <sys_creat>
What you talk about was in my initial version. But Heiko insisted on having all
wrappers together.
http://www.spinics.net/lists/linux-s390/msg11593.html
Grep your email for discussion.
>
> > > Cost wise, this seems like it all cancels out in the end, but what
> > > do I know?
> >
> > I think you know something, and I also think Heiko and other s390 guys
> > know something as well. So I'd like to listen their arguments here.
> >
> > For me spark64 way is looking reasonable only because it's really simple
> > and takes less coding. I'll try it on some branch and share here what happened.
>
> The kernel code will definitely look simpler ;). It would be good to see
> if there actually is any performance impact. Even with 16 more cycles on
> syscall entry, would they be lost in the noise? You don't need a full
> implementation, just some dummy mov x0, x0 on the entry path.
>
> --
> Catalin
next prev parent reply other threads:[~2016-05-27 0:38 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-24 0:04 [PATCH v6 00/21] ILP32 for ARM64 Yury Norov
2016-05-24 0:04 ` [PATCH 01/23] all: syscall wrappers: add documentation Yury Norov
2016-05-25 19:30 ` David Miller
2016-05-25 20:03 ` Yury Norov
2016-05-25 20:21 ` David Miller
2016-05-25 20:47 ` Arnd Bergmann
2016-05-25 20:50 ` David Miller
2016-05-25 21:01 ` Arnd Bergmann
2016-05-25 21:28 ` David Miller
2016-05-26 14:20 ` Catalin Marinas
2016-05-26 14:50 ` Szabolcs Nagy
2016-05-26 15:19 ` Catalin Marinas
2016-05-26 19:43 ` David Miller
2016-05-27 10:10 ` Catalin Marinas
2016-05-26 20:48 ` Yury Norov
2016-05-26 22:29 ` Catalin Marinas
2016-05-27 0:37 ` Yury Norov [this message]
2016-05-27 6:03 ` Heiko Carstens
2016-05-27 8:42 ` Arnd Bergmann
2016-05-27 9:30 ` Catalin Marinas
2016-05-27 10:49 ` Arnd Bergmann
2016-05-27 13:04 ` Catalin Marinas
2016-05-27 16:58 ` Yury Norov
2016-05-27 17:36 ` Catalin Marinas
2016-05-27 9:01 ` Catalin Marinas
2016-06-14 23:08 ` Yury Norov
2016-05-27 5:52 ` Heiko Carstens
2016-05-24 0:04 ` [PATCH 02/23] all: introduce COMPAT_WRAPPER option and enable it for s390 Yury Norov
2016-05-24 0:04 ` [PATCH 03/23] all: s390: move wrapper infrastructure to generic headers Yury Norov
2016-05-24 0:04 ` [PATCH 04/23] all: s390: move compat_wrappers.c from arch/s390/kernel to kernel/ Yury Norov
2016-05-24 0:04 ` [PATCH 05/23] all: wrap needed syscalls in generic unistd Yury Norov
2016-05-24 0:04 ` [PATCH 06/23] compat ABI: use non-compat openat and open_by_handle_at variants Yury Norov
2016-05-24 0:04 ` [PATCH 07/23] 32-bit ABI: introduce ARCH_32BIT_OFF_T config option Yury Norov
2016-05-24 0:04 ` [PATCH 08/23] arm64: ilp32: add documentation on the ILP32 ABI for ARM64 Yury Norov
2016-05-24 0:04 ` [PATCH 09/23] arm64: ensure the kernel is compiled for LP64 Yury Norov
2016-05-24 0:04 ` [PATCH 10/23] arm64: rename COMPAT to AARCH32_EL0 in Kconfig Yury Norov
2016-05-24 0:04 ` [PATCH 11/23] arm64:uapi: set __BITS_PER_LONG correctly for ILP32 and LP64 Yury Norov
2016-05-24 0:04 ` [PATCH 12/23] thread: move thread bits accessors to separated file Yury Norov
2016-05-24 0:04 ` [PATCH 13/23] arm64: introduce is_a32_task and is_a32_thread (for AArch32 compat) Yury Norov
2016-06-12 12:21 ` Zhangjian (Bamvor)
2016-06-12 13:08 ` Zhangjian (Bamvor)
2016-06-12 17:56 ` Yury Norov
2016-05-24 0:04 ` [PATCH 14/23] arm64: ilp32: add is_ilp32_compat_{task,thread} and TIF_32BIT_AARCH64 Yury Norov
2016-05-24 0:04 ` [PATCH 15/23] arm64: introduce binfmt_elf32.c Yury Norov
2016-05-24 0:04 ` [PATCH 16/23] arm64: ilp32: introduce binfmt_ilp32.c Yury Norov
2016-05-26 13:49 ` Zhangjian (Bamvor)
2016-05-26 21:08 ` Yury Norov
2016-06-15 0:40 ` Yury Norov
2016-06-13 3:05 ` Zhangjian (Bamvor)
2016-06-13 13:22 ` Zhangjian (Bamvor)
2016-05-24 0:04 ` [PATCH 17/23] arm64: ptrace: handle ptrace_request differently for aarch32 and ilp32 Yury Norov
2016-06-08 1:34 ` zhouchengming
2016-06-08 17:00 ` Yury Norov
2016-06-25 9:36 ` zhouchengming
2016-06-25 14:15 ` Bamvor Zhang
2016-06-27 2:09 ` zhouchengming
2016-05-24 0:04 ` [PATCH 18/23] arm64: ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it Yury Norov
2016-05-25 20:26 ` Arnd Bergmann
2016-05-24 0:04 ` [PATCH 19/23] arm64: signal: share lp64 signal routines to ilp32 Yury Norov
2016-05-24 0:04 ` [PATCH 20/23] arm64: signal32: move ilp32 and aarch32 common code to separated file Yury Norov
2016-05-24 0:04 ` [PATCH 21/23] arm64: ilp32: introduce ilp32-specific handlers for sigframe and ucontext Yury Norov
2016-06-04 11:34 ` Zhangjian (Bamvor)
2016-06-12 12:34 ` Zhangjian (Bamvor)
2016-06-12 13:12 ` Zhangjian (Bamvor)
2016-06-12 17:44 ` Yury Norov
2016-06-16 11:21 ` Zhangjian (Bamvor)
2016-06-12 12:39 ` Zhangjian (Bamvor)
2016-05-24 0:04 ` [PATCH 22/23] arm64:ilp32: add vdso-ilp32 and use for signal return Yury Norov
2016-05-24 0:04 ` [PATCH 23/23] arm64:ilp32: add ARM64_ILP32 to Kconfig Yury Norov
2016-05-25 10:42 ` [PATCH v6 00/21] ILP32 for ARM64 Szabolcs Nagy
2016-05-25 16:41 ` Yury Norov
2016-06-02 19:03 ` Yury Norov
2016-06-03 11:02 ` Szabolcs Nagy
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20160527003753.GA14247@yury-N73SV \
--to=ynorov@caviumnetworks.com \
--cc=Nathan_Lynch@mentor.com \
--cc=Prasun.Kapoor@caviumnetworks.com \
--cc=agraf@suse.de \
--cc=arnd@arndb.de \
--cc=bamvor.zhangjian@huawei.com \
--cc=broonie@kernel.org \
--cc=catalin.marinas@arm.com \
--cc=christoph.muellner@theobroma-systems.com \
--cc=davem@davemloft.net \
--cc=geert@linux-m68k.org \
--cc=heiko.carstens@de.ibm.com \
--cc=joseph@codesourcery.com \
--cc=kilobyte@angband.pl \
--cc=klimov.linux@gmail.com \
--cc=libc-alpha@sourceware.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=philipp.tomsich@theobroma-systems.com \
--cc=pinskia@gmail.com \
--cc=schwidefsky@de.ibm.com \
--cc=szabolcs.nagy@arm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).