linux: Optimize fallback 32-bit clock_gettime

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>

1 files changed, 12 insertions, 4 deletions

diff --git a/sysdeps/unix/sysv/linux/clock_gettime.c b/sysdeps/unix/sysv/linux/clock_gettime.c
index a530daef1c..d6c0b4f637 100644
--- a/sysdeps/unix/sysv/linux/clock_gettime.c
+++ b/sysdeps/unix/sysv/linux/clock_gettime.c
@@ -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;