Subject: Re: [PATCH 1/3] powerpc/tm: Abort syscalls in active transactions

From: Anshuman Khandual <[email-masked]>
Date: Fri, 20 Mar 2015 14:34:43 +0530

On 03/19/2015 10:13 AM, Sam Bobroff wrote:
> This patch changes the syscall handler to doom (tabort) active
> transactions when a syscall is made and return immediately without
> performing the syscall.
> 
> Currently, the system call instruction automatically suspends an
> active transaction which causes side effects to persist when an active
> transaction fails.
> 
> This does change the kernel's behaviour, but in a way that was
> documented as unsupported. It doesn't reduce functionality because
> syscalls will still be performed after tsuspend. It also provides a
> consistent interface and makes the behaviour of user code
> substantially the same across powerpc and platforms that do not
> support suspended transactions (e.g. x86 and s390).
> 
> Performance measurements using
> http://ozlabs.org/~anton/junkcode/null_syscall.c
> indicate the cost of a system call increases by about 0.5%.

Performance test results verified which shows an improvement of
around ~0.52% with the patch compared to without it.

[With patch]		null_syscall:     757.59 cycles     100.00%
[Without patch]]	null_syscall:     753.66 cycles     100.00%

> 
> Signed-off-by: Sam Bobroff <[email-masked]>

Tested-by: Anshuman Khandual <[email-masked]>