Re: [PATCH v2] Add x86 32 bit vDSO time function support

[Nathan Lynch <Nathan_Lynch@codesourcery.com>]

On 10/09/2014 01:31 PM, Adhemerval Zanella wrote:
> diff --git a/sysdeps/unix/sysv/linux/i386/init-first.c b/sysdeps/unix/sysv/linux/i386/init-first.c
> new file mode 100644
> index 0000000..dc3b1ba
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/i386/init-first.c
> @@ -0,0 +1,52 @@
> +/* Initialization code run first thing by the ELF startup code.  Linux/i386.
> +   Copyright (C) 2014 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/>.  */
> +
> +#ifdef SHARED
> +# include <time.h>
> +# include <sysdep.h>
> +# include <dl-vdso.h>
> +# include <libc-vdso.h>
> +
> +long int (*__vdso_clock_gettime) (clockid_t, struct timespec *)
> +  __attribute__ ((nocommon));
> +libc_hidden_proto (__vdso_clock_gettime)
> +libc_hidden_data_def (__vdso_clock_gettime)
> +
> +static long int
> +clock_gettime_syscall (clockid_t id, struct timespec *tp)
> +{
> +  INTERNAL_SYSCALL_DECL (err);
> +  return INTERNAL_SYSCALL (clock_gettime, err, 2, id, tp);
> +}
> +
> +static inline void
> +__vdso_platform_setup (void)
> +{
> +  PREPARE_VERSION (linux26, "LINUX_2.6", 61765110);

Perhaps:

     PREPARE_VERSION_KNOWN (linux26, LINUX_2_6);

(here and several other places)


> +
> +  void *p = _dl_vdso_vsym ("__vdso_clock_gettime", &linux26);
> +  if (p == NULL)
> +    p = clock_gettime_syscall;
> +  PTR_MANGLE (p);
> +  __vdso_clock_gettime = p;
> +}
> +
> +# define VDSO_SETUP __vdso_platform_setup
> +#endif
> +
> +#include <csu/init-first.c>

[...]

> diff --git a/sysdeps/unix/sysv/linux/x86/clock_gettime.c b/sysdeps/unix/sysv/linux/x86/clock_gettime.c
> new file mode 100644
> index 0000000..2547a8c
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86/clock_gettime.c
> @@ -0,0 +1,38 @@
> +/* Get the current value of a clock.  Linux/x86 version.
> +   Copyright (C) 2014 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 <libc-vdso.h>
> +
> +#ifdef SHARED
> +# define SYSCALL_GETTIME(id, tp) \
> +  ({ long int (*f) (clockid_t, struct timespec *) = __vdso_clock_gettime; \
> +  long int v_ret;							  \
> +  PTR_DEMANGLE (f);							  \
> +  v_ret = (*f) (id, tp);						  \
> +  if (INTERNAL_SYSCALL_ERROR_P (v_ret, )) {				  \
> +    __set_errno (INTERNAL_SYSCALL_ERRNO (v_ret, ));			  \
> +    v_ret = -1;								  \
> +  }									  \
> +  v_ret; })

Does introducing the dispatch through function pointer here cause a
measurable performance regression on i386 kernels which lack the VDSO?
If so, is that a concern?

When I've tried this approach on ARM, it appears to do so (around 5%
slowdown).


> +# define INTERNAL_GETTIME(id, tp) \
> +  ({ long int (*f) (clockid_t, struct timespec *) = __vdso_clock_gettime; \
> +  PTR_DEMANGLE (f);							  \
> +  (*f) (id, tp); })
> +#endif

I'm probably missing something, but I am failing to see the need for an
INTERNAL_GETTIME definition in
sysdeps/unix/sysv/linux/x86/clock_gettime.c.  I know this patch is
merely moving existing code, but sysdeps/unix/sysv/linux/clock_gettime.c
does not use INTERNAL_GETTIME, and neither does
sysdeps/unix/clock_gettime.c.

INTERNAL_GETTIME is needed for timespec_get, but I am not seeing the
need to duplicate it for clock_gettime.