Messages in this thread | | | Date | Wed, 17 Mar 2010 16:39:03 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock |
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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.
Thanks.
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