Re: [PATCH] lib/Kconfig.debug: add a comment to PROVE_LOCKING impact

[Lukasz Luba]

Hi Longman,

Thanks for the patches. I have applied them and run on ARM.
In context switch test, there is ~2% improvement
(but still ~15us instead of 5-6us).
In dhrystone: test results are the same.
There is also 2% improvement in boot time.

You can add me on CC list if you have some patches related to
this feature. I will give it a try on arm platforms.

Regards,
Lukasz

On 10/09/2018 07:29 PM, Waiman Long wrote:
> On 10/09/2018 11:39 AM, Lukasz Luba wrote:
>> This patch add some comment related to performance impact,
>> which can be really big (x3 times slower context switch).
>>
>> Signed-off-by: Lukasz Luba <l.luba@xxxxxxxxxxxxxxxxxxx>
>> ---
>>   lib/Kconfig.debug | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 4966c4f..9e67a2a3 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1088,6 +1088,9 @@ config PROVE_LOCKING
>>   	 the proof of observed correctness is also maintained for an
>>   	 arbitrary combination of these separate locking variants.
>>   
>> +	 This feature enables LOCKDEP which can harm system performance
>> +	 even x3 times.
>> +
>>   	 For more details, see Documentation/locking/lockdep-design.txt.
>>   
>>   config LOCK_STAT
>> @@ -1112,6 +1115,10 @@ config LOCK_STAT
>>   	 CONFIG_LOCK_STAT defines "contended" and "acquired" lock events.
>>   	 (CONFIG_LOCKDEP defines "acquire" and "release" events.)
>>   
>> +	 This feature enables LOCKDEP which can harm system performance
>> +	 even x3 times.
>> +	 For more details, see Documentation/locking/lockdep-design.txt.
>> +
>>   config DEBUG_RT_MUTEXES
>>   	bool "RT Mutex debugging, deadlock detection"
>>   	depends on DEBUG_KERNEL && RT_MUTEXES
>> @@ -1175,6 +1182,10 @@ config DEBUG_LOCK_ALLOC
>>   	 spin_lock_init()/mutex_init()/etc., or whether there is any lock
>>   	 held during task exit.
>>   
>> +	 This feature enables LOCKDEP which can harm system performance
>> +	 even x3 times.
>> +	 For more details, see Documentation/locking/lockdep-design.txt.
>> +
>>   config LOCKDEP
>>   	bool
>>   	depends on DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT
> 
> I don't mind seeing warning message about the performance impact of
> enabling lockdep. However, the actual slowdown depends on many factors
> including the type of systems, workloads and so on. I am a bit hesitant
> to quote an actual figure like x3 in the help text as it may not be
> correct in many cases.
> 
> BTW, my recent lockdep patch (https://lkml.org/lkml/2018/10/2/1410)
> should be able to reduce the performance impact of enabling lockdep.
> Could you try to see if that help in your case?
> 
> Cheers,
> Longman
> 
> 
> 
>