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

From: Anshuman Khandual <[email-masked]>
Date: Tue, 24 Mar 2015 09:56:39 +0530

On 03/24/2015 07:34 AM, Michael Ellerman wrote:
> On Fri, 2015-03-20 at 14:34 +0530, Anshuman Khandual wrote:
>> 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%
> 
> No that's a performance *decrease* with the patch applied. ?

Micheal, thats true. The cycle counts are more with patch being applied.
Here are some more results on mainline.

Without the patch:

        null_syscall:     754.14 cycles     100.00%
        null_syscall:     754.50 cycles     100.00%
        null_syscall:     754.17 cycles     100.00%
        null_syscall:     754.43 cycles     100.00%
        null_syscall:     754.17 cycles     100.00%
        null_syscall:     754.20 cycles     100.00%
        null_syscall:     754.47 cycles     100.00%
        null_syscall:     754.70 cycles     100.00%
        null_syscall:     754.43 cycles     100.00%
        null_syscall:     753.72 cycles     100.00%

With the patch:

        null_syscall:     756.76 cycles     100.00%
        null_syscall:     756.40 cycles     100.00%
        null_syscall:     756.46 cycles     100.00%
        null_syscall:     756.86 cycles     100.00%
        null_syscall:     756.56 cycles     100.00%
        null_syscall:     756.66 cycles     100.00%
        null_syscall:     756.86 cycles     100.00%
        null_syscall:     756.86 cycles     100.00%
        null_syscall:     757.02 cycles     100.00%
        null_syscall:     756.93 cycles     100.00%