Messages in this thread | | | Date | Thu, 18 Mar 2010 14:49:38 +0900 | From | Hitoshi Mitake <> | Subject | Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock |
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On 03/18/10 00:39, Frederic Weisbecker wrote: > On Wed, Mar 17, 2010 at 04:30:53PM +0900, Hitoshi Mitake wrote: >> 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? > > > Ah I see. > > might_lock_read() uses might_fault(), rcu, workqueues and probably > yet some others use sequences of lock_acquire/lock_release to prove > locking while there is actually no real lock operation involved, but > this is to detect dependency/balance mistakes. > > I think that these cases are easily detectable in that they never have > any lock_acquired in their scenario. So may be we can just ignore > scenarios without lock_acquired and indeed advise users not to use > PROVE_LOCKING.
Unfortunately, we cannot use this detection method. Because trylock series (e.g. spin_trylock()) only issues lock_acquire() like this,
static inline int __raw_spin_trylock(raw_spinlock_t *lock) { preempt_disable(); if (do_raw_spin_trylock(lock)) { spin_acquire(&lock->monitor, 0, 1, _RET_IP_); <- spin_acquire() only issues lock_acquire() return 1; } preempt_enable(); return 0; }
So distinguishing trylocks and lock_acquire()/lock_release() pairs from might_lock_read(), might_fault() and etc is hard.
It seems that turning off PROVE_LOCKING must be required for state machine of perf lock.
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