From: Catalin Marinas <catalin.marinas@arm.com>
To: Yury Norov <ynorov@caviumnetworks.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: Thu, 26 May 2016 23:29:45 +0100 [thread overview]
Message-ID: <20160526222943.GA16729@MBP.local> (raw)
In-Reply-To: <20160526204819.GA10274@yury-N73SV>
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>
> > 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:[~2016-05-26 22:29 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 [this message]
2016-05-27 0:37 ` Yury Norov
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
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