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

On 01/24/2019 01:06 AM, Michael Ellerman wrote:

Christophe Leroy [off-list ref] writes:

quoted

Le 12/01/2019 à 10:55, Christophe Leroy a écrit :

quoted

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