Re: [PATCH RFC 00/11] lock monitor: Separate features related tolock

[Hitoshi Mitake]

On 03/17/10 10:32, Frederic Weisbecker wrote:
> On Sun, Mar 14, 2010 at 07:13:55PM +0100, Peter Zijlstra wrote:
>> On Sun, 2010-03-14 at 19:38 +0900, Hitoshi Mitake wrote:
>>> Current lockdep is too complicated because,
>>>   * dependency validation
>>>   * statistics
>>>   * event tracing
>>> are all implemented by it.
>>> This cause problem of overhead.
>>> If user enables one of them, overhead of rests part is not avoidable.
>>> (tracing is exception. If user enables validation or stat,
>>> overhead of tracing doesn't occur.)
>>>
>>> So I suggest new subsystem "lock monitor".
>>> This is a general purpose lock event hooking mechanism.
>>>
>>> lock monitor will be enable easy implementing and running
>>> these features related to lock.
>>>
>>> And I'm hoping that lock monitor will reduce overhead of perf lock.
>>> Because lock monitor separates dependency validation and event 
tracing clearly,
>>> so calling of functions of lockdep (e.g. lock_acquire()) only for 
validation
>>> will not occur lock events.
>>>
>>> I implemented it on the branch perf/inject of Frederic's 
random-tracing tree.
>>> Because the branch is hottest place of lock and tracing :)
>>
>> OK, so I really don't like this much..
>>
>> Building a lockstat kernel (PROVE_LOCKING=n) should not have much more
>> overhead than the proposed solution, if the simple lock acquistion
>> tracking bothers you, you can do a patch to weaken that.
>>
>> I really really dislike how you add a monitor variable between
>> everything for no reason what so ever.
>>
>> You use a new rwlock_t, which is an instant fail, those things are worse
>> than useless.
>>
>> You add chained indirect calls into all lock ops, that's got to hurt.
>
>
> Well, the idea was not bad at the first glance. It was separating
> lockdep and lock events codes.
>
> But indeed, the indirect calls plus the locking are not good
> for such a fast path.
>
> There is something else, it would be nice to keep the
> lockdep_map ->  lockdep_class mapping so that we can
> do lock profiling based on classes too. So we actually
> need the lockdep code. What we don't need is the prove
> locking or the lock stats. So I guess we can have a new
> config to enable lock events and get rid of the prove
> locking / lock stat code if we don't need it.
>
>

Thanks for your comments, Peter and Frederic.

My main motivation of writing this patch series was that
some kernel codes uses lockdep functions (e.g. lock_acquire()) directly,
so perf lock gets a lot of trace events without actual locks (e.g. 
might_lock_read()).
I think that these are confusable things for users.

But I noticed that these events can be reduced by
turning off CONFIG_PROVE_LOCKING. Yeah, my patch series was pointless... :)

Should perf lock warn not to use with CONFIG_PROVE_LOCKING?

Thanks,
	Hitoshi
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