linux-kernel - Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN

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Date:   Thu, 24 Jan 2019 12:06:45 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK

Christophe Leroy <christophe.leroy@....fr> writes:
> Le 12/01/2019 à 10:55, Christophe Leroy a écrit :
>> 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