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

[Christophe Leroy]

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 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.

Christophe