Christophe Leroy [off-list ref] writes:

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?

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?

cheers