powerpc: Fix tabort usage in syscalls

  Fix usage of tabort in generated syscalls.  r0 has special meaning
when used with this instruction, thus it will not generate
persistent errors, nor return an error code.  This mitigates poor
CPU usage when performing elided critical sections.

Additionally, transactions should be aborted when entering a user
invoked syscall.  Otherwise the results of the transaction may be
undefined.

2015-08-03  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>

	* sysdeps/powerpc/powerpc32/sysdep.h (ABORT_TRANSACTION): Use
	register other than r0 for tabort, it has special meaning.
	* sysdeps/powerpc/powerpc64/sysdep.h (ABORT_TRANSACTION): Likewise
	* sysdeps/unix.sysv/linux/powerpc/syscall.S (syscall): Abort
	transaction before starting syscall.
---
  sysdeps/powerpc/powerpc32/sysdep.h        |    4 ++--
  sysdeps/powerpc/powerpc64/sysdep.h        |    4 ++--
  sysdeps/unix/sysv/linux/powerpc/syscall.S |    1 +
  3 files changed, 5 insertions(+), 4 deletions(-)
  

LGTM, thanks for checking on that.



> Em 07/08/2015, às 16:31, Paul E. Murphy <murphyp@linux.vnet.ibm.com> escreveu:
> 
> Fix usage of tabort in generated syscalls.  r0 has special meaning
> when used with this instruction, thus it will not generate
> persistent errors, nor return an error code.  This mitigates poor
> CPU usage when performing elided critical sections.

Kinda curious, could you elaborate about this performance issue?

> 
> Additionally, transactions should be aborted when entering a user
> invoked syscall.  Otherwise the results of the transaction may be
> undefined.
> 
> 2015-08-03  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>
> 
>    * sysdeps/powerpc/powerpc32/sysdep.h (ABORT_TRANSACTION): Use
>    register other than r0 for tabort, it has special meaning.
>    * sysdeps/powerpc/powerpc64/sysdep.h (ABORT_TRANSACTION): Likewise
>    * sysdeps/unix.sysv/linux/powerpc/syscall.S (syscall): Abort
>    transaction before starting syscall.
> ---
> sysdeps/powerpc/powerpc32/sysdep.h        |    4 ++--
> sysdeps/powerpc/powerpc64/sysdep.h        |    4 ++--
> sysdeps/unix/sysv/linux/powerpc/syscall.S |    1 +
> 3 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/sysdeps/powerpc/powerpc32/sysdep.h b/sysdeps/powerpc/powerpc32/sysdep.h
> index e16fe3e..ecb492a 100644
> --- a/sysdeps/powerpc/powerpc32/sysdep.h
> +++ b/sysdeps/powerpc/powerpc32/sysdep.h
> @@ -95,8 +95,8 @@ GOT_LABEL:            ;                          \
>     lwz      0,TM_CAPABLE(2);    \
>     cmpwi    0,0;        \
>     beq         1f;        \
> -    li         0,_ABORT_SYSCALL;    \
> -    tabort.  0;            \
> +    li       11,_ABORT_SYSCALL;    \
> +    tabort.  11;        \
>     .align 4;            \
> 1:
> #else
> diff --git a/sysdeps/powerpc/powerpc64/sysdep.h b/sysdeps/powerpc/powerpc64/sysdep.h
> index bf2a884..a9d37ad 100644
> --- a/sysdeps/powerpc/powerpc64/sysdep.h
> +++ b/sysdeps/powerpc/powerpc64/sysdep.h
> @@ -279,8 +279,8 @@ LT_LABELSUFFIX(name,_name_end): ; \
>     lwz      0,TM_CAPABLE(13);    \
>     cmpwi    0,0;        \
>     beq         1f;        \
> -    li         0,_ABORT_SYSCALL;    \
> -    tabort.  0;            \
> +    li       11,_ABORT_SYSCALL;    \
> +    tabort.  11;        \
>     .align 4;                   \
> 1:
> #else
> diff --git a/sysdeps/unix/sysv/linux/powerpc/syscall.S b/sysdeps/unix/sysv/linux/powerpc/syscall.S
> index 157e3e3..4477303 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/syscall.S
> +++ b/sysdeps/unix/sysv/linux/powerpc/syscall.S
> @@ -18,6 +18,7 @@
> #include <sysdep.h>
>  ENTRY (syscall)
> +    ABORT_TRANSACTION
>    mr   r0,r3
>    mr   r3,r4
>    mr   r4,r5
> -- 
> 1.7.1
>
  

On 08/08/2015 01:06 AM, Adhemerval Zanella wrote:
> LGTM, thanks for checking on that.
>
>
>
>> Em 07/08/2015, às 16:31, Paul E. Murphy <murphyp@linux.vnet.ibm.com> escreveu:
>>
>> Fix usage of tabort in generated syscalls.  r0 has special meaning
>> when used with this instruction, thus it will not generate
>> persistent errors, nor return an error code.  This mitigates poor
>> CPU usage when performing elided critical sections.
> Kinda curious, could you elaborate about this performance issue?
Profiling the case where a lock is used to protect a system call resulted in much higher
CPU usage than expected. Distilling the issue into a test case of the form:

pthread_mutex_lock(&lock);
write( fd, buf, len );
pthread_mutex_unlock(&lock);

This showed about 1 abort per lock, when it should be 1 abort per 4 locks
with the current adaptive lock settings.
  

On Fri, 2015-08-07 at 09:31 -0500, Paul E. Murphy wrote:
> Fix usage of tabort in generated syscalls.  r0 has special meaning
> when used with this instruction, thus it will not generate
> persistent errors, nor return an error code.  This mitigates poor
> CPU usage when performing elided critical sections.
> 
> Additionally, transactions should be aborted when entering a user
> invoked syscall.  Otherwise the results of the transaction may be
> undefined.
> 
> 2015-08-03  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>
> 
> 	* sysdeps/powerpc/powerpc32/sysdep.h (ABORT_TRANSACTION): Use
> 	register other than r0 for tabort, it has special meaning.
> 	* sysdeps/powerpc/powerpc64/sysdep.h (ABORT_TRANSACTION): Likewise
> 	* sysdeps/unix.sysv/linux/powerpc/syscall.S (syscall): Abort
> 	transaction before starting syscall.
> ---

LGTM and require for reliable function, Please apply
  

"Paul E. Murphy" <murphyp@linux.vnet.ibm.com> writes:

> Fix usage of tabort in generated syscalls.  r0 has special meaning
> when used with this instruction, thus it will not generate
> persistent errors, nor return an error code.  This mitigates poor
> CPU usage when performing elided critical sections.
>
> Additionally, transactions should be aborted when entering a user
> invoked syscall.  Otherwise the results of the transaction may be
> undefined.

This patch looks very good in theory, but it seems your mail client broke the
patch and git is now complaining.
Could you send a new version, please?