[12/23] linux: Add helper function to optimize 64-bit time_t fallback support
Commit Message
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
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
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).
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
@@ -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
new file mode 100644
@@ -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
new file mode 100644
@@ -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
+}