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[PATCH 11/16] linux: Optimize fallback 32-bit clock_gettime
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: libc-alpha at sourceware dot org
- Date: Tue, 17 Dec 2019 18:47:23 -0300
- Subject: [PATCH 11/16] linux: Optimize fallback 32-bit clock_gettime
- References: <20191217214728.2886-1-adhemerval.zanella@linaro.org>
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.
---
sysdeps/unix/sysv/linux/clock_gettime.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/sysdeps/unix/sysv/linux/clock_gettime.c b/sysdeps/unix/sysv/linux/clock_gettime.c
index 9be1f779e3..2a8ad0acc5 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) == 1)
+ {
# 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;
--
2.17.1