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linux: Optimize fallback 32-bit clock_gettime

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This patch avoid probing the __NR_clock_getttime64 syscall each time
__clock_gettime64 is issued on a kernel without 64 bit time support.
Once ENOSYS is obtained, only 32-bit clock_gettime are used.

The following snippet:

  clock_gettime (CLOCK_REALTIME, &(struct timespec) { 0 });
  clock_gettime (CLOCK_MONOTONIC, &(struct timespec) { 0 });
  clock_gettime (CLOCK_BOOTTIME, &(struct timespec) { 0 });
  clock_gettime (20, &(struct timespec) { 0 });

On a kernel without 64 bit time support and with vDSO support results
on the following syscalls:

  syscall_0x193(0, 0xff87ba30, [...]) = -1 ENOSYS (Function not implemented)
  clock_gettime(CLOCK_BOOTTIME, {tv_sec=927082, tv_nsec=474382032}) = 0
  clock_gettime(0x14 /* CLOCK_??? */, 0xff87b9f8) = -1 EINVAL (Invalid argument)

While on a kernel without vDSO support:

  syscall_0x193(0, 0xbec95550, 0xb6ed2000, 0x1, 0xbec95550, 0) = -1 (errno 38)
  clock_gettime(CLOCK_REALTIME, {tv_sec=1576615930, tv_nsec=638250162}) = 0
  clock_gettime(CLOCK_MONOTONIC, {tv_sec=1665478, tv_nsec=638779620}) = 0
  clock_gettime(CLOCK_BOOTTIME, {tv_sec=1675418, tv_nsec=292932704}) = 0
  clock_gettime(0x14 /* CLOCK_??? */, 0xbec95530) = -1 EINVAL (Invalid argument)

Checked on i686-linux-gnu on 4.15 kernel and on a 5.3 kernel.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
This commit is contained in:
Adhemerval Zanella 2019-12-14 10:05:44 -03:00
parent ff500a623d
commit a9091a1244
1 changed files with 12 additions and 4 deletions
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16
sysdeps/unix/sysv/linux/clock_gettime.c
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@ -43,13 +43,21 @@ __clock_gettime64 (clockid_t clock_id, struct __timespec64 *tp)
int r;
/* Old 32-bit ABI with possible 64-bit time_t support. */
# ifdef __NR_clock_gettime64
/* Avoid issue a __NR_clock_gettime64 syscall on kernels that do not
support 64-bit time_t. */
static int time64_support = 1;
if (atomic_load_relaxed (&time64_support) != 0)
{
# ifdef HAVE_CLOCK_GETTIME64_VSYSCALL
r = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
r = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
# else
r = INLINE_SYSCALL_CALL (clock_gettime64, clock_id, tp);
r = INLINE_SYSCALL_CALL (clock_gettime64, clock_id, tp);
# endif
if (r == 0 || errno != ENOSYS)
return r;
if (r == 0 || errno != ENOSYS)
return r;
atomic_store_relaxed (&time64_support, 0);
}
# endif
/* Fallback code that uses 32-bit support. */
struct timespec tp32;