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linux: Optimize fallback 32-bit clock_getres

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

On a kernel without 64 bit time support issues the syscalls:

  syscall_0x196(0, 0xffb83330, [...]) = -1 ENOSYS (Function not implemented)
  clock_getres(CLOCK_REALTIME, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_MONOTONIC, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_BOOTTIME, {tv_sec=0, tv_nsec=1}) = 0

Checked on i686-linux-gnu on 4.15 kernel.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
This commit is contained in:
Adhemerval Zanella 2019-12-16 13:50:57 -03:00
parent 0dc1a378b1
commit 92b963699a
1 changed files with 12 additions and 4 deletions
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16
sysdeps/unix/sysv/linux/clock_getres.c
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@ -42,13 +42,21 @@ __clock_getres64 (clockid_t clock_id, struct __timespec64 *res)
int r;
/* Old 32-bit ABI with possible 64-bit time_t support. */
# ifdef __NR_clock_getres_time64
/* Avoid issue a __NR_clock_getres_time64 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_GETRES64_VSYSCALL
r = INLINE_VSYSCALL (clock_getres_time64, 2, clock_id, res);
r = INLINE_VSYSCALL (clock_getres_time64, 2, clock_id, res);
# else
r = INLINE_SYSCALL_CALL (clock_getres_time64, clock_id, res);
r = INLINE_SYSCALL_CALL (clock_getres_time64, clock_id, res);
# 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 ts32;