[PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[Florian Weimer]

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(-)

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 */

base-commit: 2173173d57971d042c0ad4b281431ae127e9b5b8

Re: [PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[Michael Jeanson]

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.

Re: [PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[Michael Jeanson]

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.

Re: [PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[Arjun Shankar]

> 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.