diff mbox series

[12/23] linux: Add helper function to optimize 64-bit time_t fallback support

Message ID 20200713171025.3661832-12-adhemerval.zanella@linaro.org
State New
Headers show
Series [01/23] linux: Simplify clock_adjtime | expand

Commit Message

Adhemerval Zanella July 13, 2020, 5:10 p.m. UTC
These helper functions are used to optimize the 64-bit time_t support on
configurations that requires support for 32-bit time_t fallback
(!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises that
it does not have 64-bit time_t support, glibc will stop to try issue the
64-bit time_t syscall altogether.

For instance:

  #ifndef __NR_symbol_time64
  # define __NR_symbol_time64 __NR_symbol
  #endif
  int r;
  if (supports_time64 ())
    {
      r = INLINE_SYSCALL_CALL (symbol, ...);
      if (r == 0 || errno != ENOSYS)
        return r;

      mark_time64_unsupported ();
    }
  #ifndef __ASSUME_TIME64_SYSCALLS
  <32-bit fallback syscall>
  #endif
  return r;

On configuration with default 64-bit time_t these optimization should be
optimized away by the compiler resulting in no overhead.
---
 sysdeps/unix/sysv/linux/Makefile         |  3 +-
 sysdeps/unix/sysv/linux/time64-support.c | 23 ++++++++
 sysdeps/unix/sysv/linux/time64-support.h | 70 ++++++++++++++++++++++++
 3 files changed, 95 insertions(+), 1 deletion(-)
 create mode 100644 sysdeps/unix/sysv/linux/time64-support.c
 create mode 100644 sysdeps/unix/sysv/linux/time64-support.h

Comments

Lukasz Majewski July 14, 2020, 8:20 a.m. UTC | #1
Hi Adhemerval,

> These helper functions are used to optimize the 64-bit time_t support
> on configurations that requires support for 32-bit time_t fallback
> (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises
> that it does not have 64-bit time_t support, glibc will stop to try
> issue the 64-bit time_t syscall altogether.
> 
> For instance:
> 
>   #ifndef __NR_symbol_time64
>   # define __NR_symbol_time64 __NR_symbol
>   #endif
>   int r;
>   if (supports_time64 ())
>     {
>       r = INLINE_SYSCALL_CALL (symbol, ...);
>       if (r == 0 || errno != ENOSYS)
>         return r;
> 
>       mark_time64_unsupported ();
>     }
>   #ifndef __ASSUME_TIME64_SYSCALLS
>   <32-bit fallback syscall>
>   #endif
>   return r;
> 
> On configuration with default 64-bit time_t these optimization should
> be optimized away by the compiler resulting in no overhead.

I think such approach was proposed in the very first Y2038 conversion
patch done by Albert [1].

(By using __y2038_linux_support variable).



I think that it is an overkill (or maybe I'm not aware of some
important use cases) as new ports with __WORDSIZE==32 have already
__TIMESIZE == 64, so they use 64 bit syscalls out of the box (RISC-V,
ARC).

Moreover, systems with __WORDSIZE==32 && __TIMESIZE !=64 (like ARM)
will benefit from it only when they use:

Old Linux headers (probably with oldest kernel supported - version
3.2) and Linux version < 5.1).

All new BSPs for them will probably use kernel > 5.1 (5.4 is the newest
LTS), which will support 64 bit calls.


Link:
[1] -
https://patchwork.ozlabs.org/project/glibc/patch/20180919073553.28153-1-albert.aribaud@3adev.fr/


> ---
>  sysdeps/unix/sysv/linux/Makefile         |  3 +-
>  sysdeps/unix/sysv/linux/time64-support.c | 23 ++++++++
>  sysdeps/unix/sysv/linux/time64-support.h | 70
> ++++++++++++++++++++++++ 3 files changed, 95 insertions(+), 1
> deletion(-) create mode 100644
> sysdeps/unix/sysv/linux/time64-support.c create mode 100644
> sysdeps/unix/sysv/linux/time64-support.h
> 
> diff --git a/sysdeps/unix/sysv/linux/Makefile
> b/sysdeps/unix/sysv/linux/Makefile index 1932ccf7df..75c60a58f3 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -60,7 +60,8 @@ sysdep_routines += adjtimex clone umount umount2
> readahead sysctl \ personality epoll_wait tee vmsplice splice \
>  		   open_by_handle_at mlock2 pkey_mprotect pkey_set
> pkey_get \ timerfd_gettime timerfd_settime prctl \
> -		   process_vm_readv process_vm_writev clock_adjtime
> +		   process_vm_readv process_vm_writev clock_adjtime \
> +		   time64-support
>  
>  CFLAGS-gethostid.c = -fexceptions
>  CFLAGS-tee.c = -fexceptions -fasynchronous-unwind-tables
> diff --git a/sysdeps/unix/sysv/linux/time64-support.c
> b/sysdeps/unix/sysv/linux/time64-support.c new file mode 100644
> index 0000000000..9f03c9adda
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/time64-support.c
> @@ -0,0 +1,23 @@
> +/* Auxiliary definitions for 64-bit time_t support.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be
> useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <time64-support.h>
> +
> +#ifndef __ASSUME_TIME64_SYSCALLS
> +int __time64_support = 1;
> +#endif
> diff --git a/sysdeps/unix/sysv/linux/time64-support.h
> b/sysdeps/unix/sysv/linux/time64-support.h new file mode 100644
> index 0000000000..a997042fea
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/time64-support.h
> @@ -0,0 +1,70 @@
> +/* Auxiliary definitions for 64-bit time_t support.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be
> useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <stdbool.h>
> +#include <atomic.h>
> +
> +/* These helper functions are used to optimize the 64-bit time_t
> support on
> +   configurations that requires support for 32-bit time_t fallback
> +   (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel
> advertises that
> +   it does not have 64-bit time_t support, glibc will stop to try
> issue the
> +   64-bit time_t syscall altogether.
> +
> +   For instance:
> +
> +     #ifndef __NR_symbol_time64
> +     # define __NR_symbol_time64 __NR_symbol
> +     #endif
> +     int r;
> +     if (supports_time64 ())
> +       {
> +         r = INLINE_SYSCALL_CALL (symbol, ...);
> +         if (r == 0 || errno != ENOSYS)
> +	   return r;
> +
> +         mark_time64_unsupported ();
> +       }
> +     #ifndef __ASSUME_TIME64_SYSCALLS
> +       <32-bit fallback syscall>
> +     #endif
> +     return r;
> +
> +   On configuration with default 64-bit time_t these optimization
> should be
> +   optimized away by the compiler resulting in no overhead.  */
> +
> +#ifndef __ASSUME_TIME64_SYSCALLS
> +extern int __time64_support attribute_hidden;
> +#endif
> +
> +static inline bool
> +supports_time64 (void)
> +{
> +#ifdef __ASSUME_TIME64_SYSCALLS
> +  return true;
> +#else
> +  return atomic_load_relaxed (&__time64_support) != 0;
> +#endif
> +}
> +
> +static inline void
> +mark_time64_unsupported (void)
> +{
> +#ifndef __ASSUME_TIME64_SYSCALLS
> +  atomic_store_relaxed (&__time64_support, 0);
> +#endif
> +}




Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de
Adhemerval Zanella July 16, 2020, 1:15 p.m. UTC | #2
On 14/07/2020 05:20, Lukasz Majewski wrote:
> Hi Adhemerval,
> 
>> These helper functions are used to optimize the 64-bit time_t support
>> on configurations that requires support for 32-bit time_t fallback
>> (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises
>> that it does not have 64-bit time_t support, glibc will stop to try
>> issue the 64-bit time_t syscall altogether.
>>
>> For instance:
>>
>>   #ifndef __NR_symbol_time64
>>   # define __NR_symbol_time64 __NR_symbol
>>   #endif
>>   int r;
>>   if (supports_time64 ())
>>     {
>>       r = INLINE_SYSCALL_CALL (symbol, ...);
>>       if (r == 0 || errno != ENOSYS)
>>         return r;
>>
>>       mark_time64_unsupported ();
>>     }
>>   #ifndef __ASSUME_TIME64_SYSCALLS
>>   <32-bit fallback syscall>
>>   #endif
>>   return r;
>>
>> On configuration with default 64-bit time_t these optimization should
>> be optimized away by the compiler resulting in no overhead.
> 
> I think such approach was proposed in the very first Y2038 conversion
> patch done by Albert [1].
> 
> (By using __y2038_linux_support variable).

Thanks I didn't see this in fact.

> 
> 
> 
> I think that it is an overkill (or maybe I'm not aware of some
> important use cases) as new ports with __WORDSIZE==32 have already
> __TIMESIZE == 64, so they use 64 bit syscalls out of the box (RISC-V,
> ARC).

It should be a noop on newer 32-bit ports with 64-bit time_t, so no
overhead (as for any other 64-bit time_t  only ABI).

> 
> Moreover, systems with __WORDSIZE==32 && __TIMESIZE !=64 (like ARM)
> will benefit from it only when they use:
> 
> Old Linux headers (probably with oldest kernel supported - version
> 3.2) and Linux version < 5.1).
> 
> All new BSPs for them will probably use kernel > 5.1 (5.4 is the newest
> LTS), which will support 64 bit calls.

That's why I using this optimization only on specific symbols, I don't
foresee that it would be common for time64 binaries on old 32-bit abi 
with 32-bit time_t support would be deployed on pre v5.1 kernels.

In any case, I still think that this should be quite simple optimization
(the exempla above only adds about a couple of extra lines).
Lukasz Majewski July 17, 2020, 7:11 a.m. UTC | #3
Hi Adhemerval,

> On 14/07/2020 05:20, Lukasz Majewski wrote:
> > Hi Adhemerval,
> >   
> >> These helper functions are used to optimize the 64-bit time_t
> >> support on configurations that requires support for 32-bit time_t
> >> fallback (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel
> >> advertises that it does not have 64-bit time_t support, glibc will
> >> stop to try issue the 64-bit time_t syscall altogether.
> >>
> >> For instance:
> >>
> >>   #ifndef __NR_symbol_time64
> >>   # define __NR_symbol_time64 __NR_symbol
> >>   #endif
> >>   int r;
> >>   if (supports_time64 ())
> >>     {
> >>       r = INLINE_SYSCALL_CALL (symbol, ...);
> >>       if (r == 0 || errno != ENOSYS)
> >>         return r;
> >>
> >>       mark_time64_unsupported ();
> >>     }
> >>   #ifndef __ASSUME_TIME64_SYSCALLS
> >>   <32-bit fallback syscall>
> >>   #endif
> >>   return r;
> >>
> >> On configuration with default 64-bit time_t these optimization
> >> should be optimized away by the compiler resulting in no overhead.
> >>  
> > 
> > I think such approach was proposed in the very first Y2038
> > conversion patch done by Albert [1].
> > 
> > (By using __y2038_linux_support variable).  
> 
> Thanks I didn't see this in fact.
> 
> > 
> > 
> > 
> > I think that it is an overkill (or maybe I'm not aware of some
> > important use cases) as new ports with __WORDSIZE==32 have already
> > __TIMESIZE == 64, so they use 64 bit syscalls out of the box
> > (RISC-V, ARC).  
> 
> It should be a noop on newer 32-bit ports with 64-bit time_t, so no
> overhead (as for any other 64-bit time_t  only ABI).

Ok.

> 
> > 
> > Moreover, systems with __WORDSIZE==32 && __TIMESIZE !=64 (like ARM)
> > will benefit from it only when they use:
> > 
> > Old Linux headers (probably with oldest kernel supported - version
> > 3.2) and Linux version < 5.1).
> > 
> > All new BSPs for them will probably use kernel > 5.1 (5.4 is the
> > newest LTS), which will support 64 bit calls.  
> 
> That's why I using this optimization only on specific symbols, I don't
> foresee that it would be common for time64 binaries on old 32-bit abi 
> with 32-bit time_t support would be deployed on pre v5.1 kernels.
> 
> In any case, I still think that this should be quite simple
> optimization (the exempla above only adds about a couple of extra
> lines).
> 

My _only_ concern is if the work done on this optimization will have
enough relevant use cases to pay off.


Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de
diff mbox series

Patch

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 1932ccf7df..75c60a58f3 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -60,7 +60,8 @@  sysdep_routines += adjtimex clone umount umount2 readahead sysctl \
 		   personality epoll_wait tee vmsplice splice \
 		   open_by_handle_at mlock2 pkey_mprotect pkey_set pkey_get \
 		   timerfd_gettime timerfd_settime prctl \
-		   process_vm_readv process_vm_writev clock_adjtime
+		   process_vm_readv process_vm_writev clock_adjtime \
+		   time64-support
 
 CFLAGS-gethostid.c = -fexceptions
 CFLAGS-tee.c = -fexceptions -fasynchronous-unwind-tables
diff --git a/sysdeps/unix/sysv/linux/time64-support.c b/sysdeps/unix/sysv/linux/time64-support.c
new file mode 100644
index 0000000000..9f03c9adda
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/time64-support.c
@@ -0,0 +1,23 @@ 
+/* Auxiliary definitions for 64-bit time_t support.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <time64-support.h>
+
+#ifndef __ASSUME_TIME64_SYSCALLS
+int __time64_support = 1;
+#endif
diff --git a/sysdeps/unix/sysv/linux/time64-support.h b/sysdeps/unix/sysv/linux/time64-support.h
new file mode 100644
index 0000000000..a997042fea
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/time64-support.h
@@ -0,0 +1,70 @@ 
+/* Auxiliary definitions for 64-bit time_t support.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdbool.h>
+#include <atomic.h>
+
+/* These helper functions are used to optimize the 64-bit time_t support on
+   configurations that requires support for 32-bit time_t fallback
+   (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises that
+   it does not have 64-bit time_t support, glibc will stop to try issue the
+   64-bit time_t syscall altogether.
+
+   For instance:
+
+     #ifndef __NR_symbol_time64
+     # define __NR_symbol_time64 __NR_symbol
+     #endif
+     int r;
+     if (supports_time64 ())
+       {
+         r = INLINE_SYSCALL_CALL (symbol, ...);
+         if (r == 0 || errno != ENOSYS)
+	   return r;
+
+         mark_time64_unsupported ();
+       }
+     #ifndef __ASSUME_TIME64_SYSCALLS
+       <32-bit fallback syscall>
+     #endif
+     return r;
+
+   On configuration with default 64-bit time_t these optimization should be
+   optimized away by the compiler resulting in no overhead.  */
+
+#ifndef __ASSUME_TIME64_SYSCALLS
+extern int __time64_support attribute_hidden;
+#endif
+
+static inline bool
+supports_time64 (void)
+{
+#ifdef __ASSUME_TIME64_SYSCALLS
+  return true;
+#else
+  return atomic_load_relaxed (&__time64_support) != 0;
+#endif
+}
+
+static inline void
+mark_time64_unsupported (void)
+{
+#ifndef __ASSUME_TIME64_SYSCALLS
+  atomic_store_relaxed (&__time64_support, 0);
+#endif
+}