Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK

From: Christophe Leroy
Date: Thu Jan 24 2019 - 10:58:46 EST




Le 24/01/2019 Ã 16:01, Christophe Leroy a ÃcritÂ:


Le 24/01/2019 Ã 10:43, Christophe Leroy a ÃcritÂ:


On 01/24/2019 01:06 AM, Michael Ellerman wrote:
Christophe Leroy <christophe.leroy@xxxxxx> writes:
Le 12/01/2019 Ã 10:55, Christophe Leroy a ÃcritÂ:
The purpose of this serie is to activate CONFIG_THREAD_INFO_IN_TASK which
moves the thread_info into task_struct.

Moving thread_info into task_struct has the following advantages:
- It protects thread_info from corruption in the case of stack
overflows.
- Its address is harder to determine if stack addresses are
leaked, making a number of attacks more difficult.

I ran null_syscall and context_switch benchmark selftests and the result
is surprising. There is slight degradation in context_switch and a
significant one on null_syscall:

Without the serie:

~# chrt -f 98 ./context_switch --no-altivec --no-vector --no-fp
55542
55562
55564
55562
55568
...

~# ./null_syscall
ÂÂÂÂ 2546.71 nsÂÂÂÂ 336.17 cycles


With the serie:

~# chrt -f 98 ./context_switch --no-altivec --no-vector --no-fp
55138
55142
55152
55144
55142

~# ./null_syscall
ÂÂÂÂ 3479.54 nsÂÂÂÂ 459.30 cycles

So 0,8% less context switches per second and 37% more time for one syscall ?

Any idea ?

What platform is that on?

It is on the 8xx

On the 83xx, I have a slight improvment:

Without the serie:

root@vgoippro:~# ./null_syscall
921.44 ns 307.15 cycles

With the serie:

root@vgoippro:~# ./null_syscall
918.78 ns 306.26 cycles

Christophe



On 64-bit we have to turn one mtmsrd into two and that's obviously a
slow down. But I don't see that you've done anything similar in 32-bit
code.

I assume it's patch 8 that causes the slow down?

I have not digged into it yet, but why patch 8 ?


The increase of null_syscall duration happens with patch 5 when we activate CONFIG_THREAD_INFO_IN_TASK.