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

From: Christophe Leroy
Date: Thu Jan 24 2019 - 04:43:54 EST




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
2546.71 ns 336.17 cycles


With the serie:

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

~# ./null_syscall
3479.54 ns 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 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 ?


I run null_syscall with perf, and I get the following. Can we conclude on something with that ?

Without the serie:

# Overhead Samples Command Shared Object Symbol
# ........ ............ ............ ................. ........................................
#
32.95% 46375 null_syscall [kernel.kallsyms] [k] DoSyscall
23.64% 33275 null_syscall [kernel.kallsyms] [k] __task_pid_nr_ns
15.47% 21778 null_syscall libc-2.23.so [.] __GI___getppid
8.92% 12556 null_syscall [kernel.kallsyms] [k] __rcu_read_unlock
5.69% 8014 null_syscall [kernel.kallsyms] [k] sys_getppid
4.01% 5643 null_syscall [kernel.kallsyms] [k] __rcu_read_lock
3.67% 5166 null_syscall [kernel.kallsyms] [k] syscall_dotrace_cont
2.52% 3542 null_syscall null_syscall [.] main

With the serie:

30.04% 56337 null_syscall [kernel.kallsyms] [k] DoSyscall
13.89% 26060 null_syscall [kernel.kallsyms] [k] __rcu_read_unlock
13.36% 25062 null_syscall libc-2.23.so [.] __GI___getppid
12.73% 23872 null_syscall [kernel.kallsyms] [k] __task_pid_nr_ns
11.21% 21033 null_syscall [kernel.kallsyms] [k] sys_getppid
8.24% 15457 null_syscall [kernel.kallsyms] [k] syscall_dotrace_cont
4.38% 8217 null_syscall [kernel.kallsyms] [k] ret_from_syscall
2.54% 4773 null_syscall null_syscall [.] main


Christophe