[11/16] linux: Optimize fallback 32-bit clock_gettime

Message ID 20191217214728.2886-11-adhemerval.zanella@linaro.org
State Superseded
Headers show
Series
  • [v2,01/16] linux: Fix vDSO macros build with time64 interfaces
Related show

Commit Message

Adhemerval Zanella Dec. 17, 2019, 9:47 p.m.
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(-)

-- 
2.17.1

Comments

Andreas Schwab Dec. 18, 2019, 8:53 a.m. | #1
On Dez 17 2019, Adhemerval Zanella wrote:

> 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)


Just != 0 should be enough.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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"And now for something completely different."
Adhemerval Zanella Dec. 18, 2019, 2:58 p.m. | #2
On 18/12/2019 05:53, Andreas Schwab wrote:
> On Dez 17 2019, Adhemerval Zanella wrote:

> 

>> 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)

> 

> Just != 0 should be enough.

> 

> Andreas.


Ack.

Patch

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;