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

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Date:   Thu, 24 Jan 2019 16:01:59 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK



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@....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 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