Re: [PATCH v2 07/10] Use clock_gettime to implement timespec_get.

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 28/08/2019 12:32, Zack Weinberg wrote:
> timespec_get is the same function as clock_gettime, with an obnoxious
> coating of NIH painted on it by the ISO C committee.  In addition to
> the rename, it takes its arguments in a different order, it returns 0
> on *failure* or a positive number on *success*, and it requires that
> all of its TIME_* constants be positive.  This last means we cannot
> directly reuse the existing CLOCK_* constants for it, because
> those have been allocated starting with CLOCK_REALTIME = 0 on all
> existing platforms.
> 
> As such I think the most sensible GNUish policy for this function is
> that we will provide it, but we will not recommend its use, we will
> not go out of our way to make it efficient, and we will not attempt to
> extend the set of TIME_* constants beyond the ones defined in ISO
> C (and in POSIX, if it ever appears in POSIX).

I don't fully agree we should not optimize because of the mentioned reasons.
Like or not, it is the C11 standard function to get the timer in a system
independent way and I would expect that once more systems implement it
portable programs will rely on it (it seems that even Windows now support
it).

And I think we can still optimize it without adding much complexity on the
code base. One idea is to simple provide a system specific internal
header that on Linux will issue the INTERNAL_VSYSCAL as an inline
function. Something like:

--
#ifdef HAVE_CLOCK_GETTIME_VSYSCALL
# define HAVE_VSYSCALL
#endif
#include <sysdep-vdso.h>

static inline int
internal_clock_gettime (clockid_t clk_id, struct timespec *tp)
{
  return INLINE_VSYSCALL (clock_gettime, 2, clock_id, tp);
}
--

I think for timer function *specifically*, some users expects it has a low
latency for some usage patterns and I think we should provide if it is
feasible.

> 
> This patch simply promotes the sysdeps/posix implementation to
> universal, and removes the Linux-specific implementation, whose
> apparent reason for existing was to cut out one function call's worth
> of overhead.  Oh well.
> 
> 	* sysdeps/posix/timespec_get.c
> 	* sysdeps/unix/sysv/linux/timespec_get.c: Delete file.
> 	* time/timespec_get.c: No longer a stub.  Replace implementation
> 	with the code formerly in sysdeps/posix/timespec_get.c.
> ---
>  sysdeps/posix/timespec_get.c           | 32 ------------------
>  sysdeps/unix/sysv/linux/timespec_get.c | 46 --------------------------
>  time/timespec_get.c                    | 14 +++-----
>  3 files changed, 4 insertions(+), 88 deletions(-)
>  delete mode 100644 sysdeps/posix/timespec_get.c
>  delete mode 100644 sysdeps/unix/sysv/linux/timespec_get.c
> 
> diff --git a/sysdeps/posix/timespec_get.c b/sysdeps/posix/timespec_get.c
> deleted file mode 100644
> index 781a624c7d..0000000000
> --- a/sysdeps/posix/timespec_get.c
> +++ /dev/null
> @@ -1,32 +0,0 @@
> -/* timespec_get -- C11 interface to sample a clock.  Generic POSIX.1 version.
> -   Copyright (C) 2013-2019 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/>.  */
> -
> -#include <time.h>
> -
> -
> -/* Set TS to calendar time based in time base BASE.  */
> -int
> -timespec_get (struct timespec *ts, int base)
> -{
> -  if (base == TIME_UTC)
> -    {
> -      __clock_gettime (CLOCK_REALTIME, ts);
> -      return base;
> -    }
> -  return 0;
> -}

Ok.

> diff --git a/sysdeps/unix/sysv/linux/timespec_get.c b/sysdeps/unix/sysv/linux/timespec_get.c
> deleted file mode 100644
> index 52080ddf08..0000000000
> --- a/sysdeps/unix/sysv/linux/timespec_get.c
> +++ /dev/null
> @@ -1,46 +0,0 @@
> -/* Copyright (C) 2011-2019 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.
> -
> -   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/>.  */
> -
> -#include <time.h>
> -#include <sysdep.h>
> -#include <errno.h>
> -
> -#ifdef HAVE_CLOCK_GETTIME_VSYSCALL
> -# define HAVE_VSYSCALL
> -#endif
> -#include <sysdep-vdso.h>
> -
> -/* Set TS to calendar time based in time base BASE.  */
> -int
> -timespec_get (struct timespec *ts, int base)
> -{
> -  switch (base)
> -    {
> -      int res;
> -      INTERNAL_SYSCALL_DECL (err);
> -    case TIME_UTC:
> -      res = INTERNAL_VSYSCALL (clock_gettime, err, 2, CLOCK_REALTIME, ts);
> -      if (INTERNAL_SYSCALL_ERROR_P (res, err))
> -	return 0;
> -      break;
> -
> -    default:
> -      return 0;
> -    }
> -
> -  return base;
> -}

As before.

> diff --git a/time/timespec_get.c b/time/timespec_get.c
> index c9e58be5d8..5ac765add1 100644
> --- a/time/timespec_get.c
> +++ b/time/timespec_get.c
> @@ -22,16 +22,10 @@
>  int
>  timespec_get (struct timespec *ts, int base)
>  {
> -  switch (base)
> +  if (base == TIME_UTC)
>      {
> -    case TIME_UTC:
> -      /* Not supported.  */
> -      return 0;
> -
> -    default:
> -      return 0;
> +      __clock_gettime (CLOCK_REALTIME, ts);
> +      return base;
>      }
> -
> -  return base;
> +  return 0;
>  }
> -stub_warning (timespec_get)
> 

Ok.