[v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)
Checks
| Context |
Check |
Description |
| redhat-pt-bot/TryBot-apply_patch |
success
|
Patch applied to master at the time it was sent
|
| redhat-pt-bot/TryBot-32bit |
success
|
Build for i686
|
| linaro-tcwg-bot/tcwg_glibc_build--master-arm |
success
|
Testing passed
|
| linaro-tcwg-bot/tcwg_glibc_check--master-arm |
success
|
Testing passed
|
| linaro-tcwg-bot/tcwg_glibc_build--master-aarch64 |
success
|
Testing passed
|
| linaro-tcwg-bot/tcwg_glibc_check--master-aarch64 |
success
|
Testing passed
|
Commit Message
Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
Unconditionally use a 32-byte rseq area"). After that, it was
not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
compilation that provided a definition. This commit always checks
the rseq area for CPU number information before using the other
approaches.
This adds an unnecessary (but well-predictable) branch on
architectures which do not define RSEQ_SIG, but its cost is small
compared to the system call. Most architectures that have vDSO
acceleration for getcpu also have rseq support.
Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
Fixes: 1d350aa06091211863e41169729cee1bca39f72f
---
v3: Remove #ifdef RSEQ_SIG instead of making it defined.
sysdeps/unix/sysv/linux/sched_getcpu.c | 8 --------
1 file changed, 8 deletions(-)
base-commit: 2173173d57971d042c0ad4b281431ae127e9b5b8
Comments
On 2024-03-13 15:28, Florian Weimer wrote:
> Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
> in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
> Unconditionally use a 32-byte rseq area"). After that, it was
> not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
> compilation that provided a definition. This commit always checks
> the rseq area for CPU number information before using the other
> approaches.
>
> This adds an unnecessary (but well-predictable) branch on
> architectures which do not define RSEQ_SIG, but its cost is small
> compared to the system call. Most architectures that have vDSO
> acceleration for getcpu also have rseq support.
>
> Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
> Fixes: 1d350aa06091211863e41169729cee1bca39f72f
Would adding back the '#include <sys/rseq.h>' in nptl/descr.h have any adverse
consequences?
It would restore the original behavior without any effect on architectures
which do not define RSEQ_SIG.
On 2024-03-13 16:51, Michael Jeanson wrote:
> On 2024-03-13 15:28, Florian Weimer wrote:
>> Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
>> in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
>> Unconditionally use a 32-byte rseq area"). After that, it was
>> not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
>> compilation that provided a definition. This commit always checks
>> the rseq area for CPU number information before using the other
>> approaches.
>>
>> This adds an unnecessary (but well-predictable) branch on
>> architectures which do not define RSEQ_SIG, but its cost is small
>> compared to the system call. Most architectures that have vDSO
>> acceleration for getcpu also have rseq support.
>>
>> Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
>> Fixes: 1d350aa06091211863e41169729cee1bca39f72f
>
> Would adding back the '#include <sys/rseq.h>' in nptl/descr.h have any adverse
> consequences?
>
> It would restore the original behavior without any effect on architectures
> which do not define RSEQ_SIG.
Just saw the V2 discussion, please ignore this.
> Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
> in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
> Unconditionally use a 32-byte rseq area"). After that, it was
> not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
> compilation that provided a definition. This commit always checks
> the rseq area for CPU number information before using the other
> approaches.
>
> This adds an unnecessary (but well-predictable) branch on
> architectures which do not define RSEQ_SIG, but its cost is small
> compared to the system call. Most architectures that have vDSO
> acceleration for getcpu also have rseq support.
This looks good to me.
Reviewed-by: Arjun Shankar <arjun@redhat.com>
>
> Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
OK. This commit removed the #include for sys/rseq.h.
> Fixes: 1d350aa06091211863e41169729cee1bca39f72f
OK. This commit made sched_getcpu's implementation depend on the
definition of RSEQ_SIG.
>
> ---
> v3: Remove #ifdef RSEQ_SIG instead of making it defined.
> sysdeps/unix/sysv/linux/sched_getcpu.c | 8 --------
> 1 file changed, 8 deletions(-)
>
> diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
> index dfb884568d..72a3360550 100644
> --- a/sysdeps/unix/sysv/linux/sched_getcpu.c
> +++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
> @@ -33,17 +33,9 @@ vsyscall_sched_getcpu (void)
> return r == -1 ? r : cpu;
> }
>
> -#ifdef RSEQ_SIG
> int
> sched_getcpu (void)
> {
> int cpu_id = THREAD_GETMEM_VOLATILE (THREAD_SELF, rseq_area.cpu_id);
> return __glibc_likely (cpu_id >= 0) ? cpu_id : vsyscall_sched_getcpu ();
> }
> -#else /* RSEQ_SIG */
> -int
> -sched_getcpu (void)
> -{
> - return vsyscall_sched_getcpu ();
> -}
> -#endif /* RSEQ_SIG */
OK. Get rid of the "backup" implementation. Now, on machines without
support, cpu_id will first be set to RSEQ_CPU_ID_REGISTRATION_FAILED,
which is negative. Then we will call vsyscall_sched_getcpu instead.
@@ -33,17 +33,9 @@ vsyscall_sched_getcpu (void)
return r == -1 ? r : cpu;
}
-#ifdef RSEQ_SIG
int
sched_getcpu (void)
{
int cpu_id = THREAD_GETMEM_VOLATILE (THREAD_SELF, rseq_area.cpu_id);
return __glibc_likely (cpu_id >= 0) ? cpu_id : vsyscall_sched_getcpu ();
}
-#else /* RSEQ_SIG */
-int
-sched_getcpu (void)
-{
- return vsyscall_sched_getcpu ();
-}
-#endif /* RSEQ_SIG */