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Subject: Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK
From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Michael Ellerman <mpe@ellerman.id.au>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>, Nicholas Piggin <npiggin@gmail.com>
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Date: Thu, 24 Jan 2019 16:58:41 +0100
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Le 24/01/2019 à 16:01, Christophe Leroy a écrit :
> 
> 
> Le 24/01/2019 à 10:43, Christophe Leroy a écrit :
>>
>>
>> On 01/24/2019 01:06 AM, Michael Ellerman wrote:
>>> Christophe Leroy <christophe.leroy@c-s.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?
>>
>> It is on the 8xx

On the 83xx, I have a slight improvment:

Without the serie:

root@vgoippro:~# ./null_syscall
     921.44 ns     307.15 cycles

With the serie:

root@vgoippro:~# ./null_syscall
     918.78 ns     306.26 cycles

Christophe

>>
>>>
>>> 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.
>