Re: [Patch] Use VDSO interface for gettimeofday on aarch64

[Andrew Pinski <pinskia at gmail dot com>]

On Thu, May 10, 2018 at 9:05 PM, Siddhesh Poyarekar <siddhesh@gotplt.org> wrote:
> On 05/10/2018 11:30 PM, Steve Ellcey wrote:
>>
>>
>> This is a Aarch64 version of gettimeofday that uses the VDSO interface
>> when it is available.  I did a test with 100000000 gettimeofday calls
>> on a T88 and the time went from 7.1 seconds to 5.5 seconds.   I also
>> ran the glibc testsuite and I did not get any regressions.
>>
>> OK to checkin?
>
>
> Can you please add a benchmark in benchtests that measures this?  I think
> you could walk the ifuncs and compare with the fallback similar to what we
> do with string functions.

That does not measure the difference here.  We were already using the
VDSO before; just differently.  Before we had a PLT that calls into
the libc and would do then an check on the VDSO and then (another)
indirect call (to the VDSO).  After we just have an PLT (indirect)
call into the VDSO directly.  Basically saving the check and an extra
indirect call (the indirect call can be costly if not predicted
correctly).

Walking the ifuncs won't benchmark the difference that is gotten here.

>
> Also, do you see an impact of this on any standard benchmarks such as
> phoronix/openbenchmarking or SPEC?

You will see the biggest benefit from Apache Bench and running Apache
as the web server (maybe any other webserver which calls gettimeofday
a lot as you are producing a date for each socket connection).   And
yes this is considered a standard benchmark when it comes to CDN.

Thanks,
Andrew


>
> Thanks,
> Siddhesh
>
>
>> Steve Ellcey
>> sellcey@cavium.com
>>
>>
>> 2018-05-10  Steve Ellcey  <sellcey@caviumnetworks.com>
>>
>>         * sysdeps/unix/sysv/linux/aarch64/gettimeofday.c: New file.
>>
>>
>> diff --git a/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
>> b/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
>> index e69de29..973b6ff 100644
>> --- a/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
>> +++ b/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
>> @@ -0,0 +1,69 @@
>> +/* Copyright (C) 2018 Free Software Foundation, Inc.
>> +
>> +   This file is part of the GNU C Library.
>> +
>> +   The GNU C Library is free software; you can redistribute it and/or
>> +   modify it under the terms of the GNU Lesser General Public License as
>> +   published by the Free Software Foundation; either version 2.1 of the
>> +   License, or (at your option) any later version.
>> +
>> +   The GNU C Library is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> +   Lesser General Public License for more details.
>> +
>> +   You should have received a copy of the GNU Lesser General Public
>> +   License along with the GNU C Library; if not, see
>> +   <http://www.gnu.org/licenses/>.  */
>> +
>> +/* Get the current time of day and timezone information,
>> +   putting it into *tv and *tz.  If tz is null, *tz is not filled.
>> +   Returns 0 on success, -1 on errors.  */
>> +
>> +#ifdef SHARED
>> +
>> +# define __gettimeofday __redirect___gettimeofday
>> +# include <sys/time.h>
>> +# undef __gettimeofday
>> +# define HAVE_VSYSCALL
>> +# include <dl-vdso.h>
>> +# include <sysdep-vdso.h>
>> +
>> +static int
>> +__gettimeofday_syscall (struct timeval *tv, struct timezone *tz)
>> +{
>> +  return INLINE_VSYSCALL (gettimeofday, 2, tv, tz);
>> +}
>> +
>> +/* PREPARE_VERSION will need an __LP64__ ifdef when ILP32 support
>> +   goes in.  See _libc_vdso_platform_setup in
>> +   sysdeps/unix/sysv/linux/aarch64/init-first.c.  */
>> +
>> +# undef INIT_ARCH
>> +# define INIT_ARCH() \
>> +          PREPARE_VERSION (linux_version, "LINUX_2.6.39", 123718537); \
>> +          void *vdso_gettimeofday = \
>> +            _dl_vdso_vsym ("__kernel_gettimeofday", &linux_version);
>> +
>> +libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
>> +                    vdso_gettimeofday ?: (void *) __gettimeofday_syscall)
>> +
>> +# undef libc_hidden_def
>> +# define libc_hidden_def(name)                               \
>> +  __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday,  \
>> +               __gettimeofday_syscall);
>> +
>> +#else
>> +
>> +# include <sys/time.h>
>> +# include <sysdep.h>
>> +int
>> +__gettimeofday (struct timeval *tv, struct timezone *tz)
>> +{
>> +  return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
>> +}
>> +#endif
>> +
>> +libc_hidden_def (__gettimeofday)
>> +weak_alias (__gettimeofday, gettimeofday)
>> +libc_hidden_weak (gettimeofday)
>>
>