Messages in this thread | | | Date | Sat, 6 Feb 2010 12:12:12 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 10/11] tracing/perf: Fix lock events recursions in the fast path |
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On Fri, Feb 05, 2010 at 02:01:55PM +0100, Peter Zijlstra wrote: > On Fri, 2010-02-05 at 13:12 +0100, Peter Zijlstra wrote: > > On Fri, 2010-02-05 at 13:10 +0100, Peter Zijlstra wrote: > > > On Fri, 2010-02-05 at 11:49 +0100, Ingo Molnar wrote: > > > > > > > > > That said, I'm not at all happy about removing lockdep annotations to make > > > > > the tracer faster, that's really counter productive. > > > > > > > > Are there no dynamic techniques that could be used here? > > > > > > > > Lockdep obviously wants maximum instrumentation coverage - performance be > > > > damned. > > > > > > > > Lock profiling/tracing/visualization wants the minimum subset of events it is > > > > interested in - everything else is unnecessary overhead. > > > > > > Well, they could start by moving the tracepoint inside the lockdep > > > recursion check. > > > > IIRC the reason its now outside is that you'd loose tracepoint on > > lockdep_off() usage, but having the tracer folks help on removing any > > such usage is of course a good thing. > > > > The usage thereof in nmi_enter() doesn't seem like a problem, since > > you're not supposed to be using locks from nmi context anyway, more so, > > I'd not be adverse to putting BUG_ON(in_nmi()) in every lockdep hook. > > Another nasty side effect is that it (lockdep recursion) isn't IRQ aware > in that we don't do any tracking for IRQ's that hit while we're doing > lockdep. We can fix that using a recursion context like we did for perf, > that would actually improve lockdep itself too.
Yep, I agree with you. With the lockdep recursion check fixed to be subtle enough for that + the lock events under lockdep recursion checks, it fixes the situation while keeping the lockdep coverage in perf tracing path for other cases.
I will start by adressing this.
That said, I think this is good for a first step, but we can't continue to force the lock events -> lockdep dependency in the long term. We can't have a serious lock profiling if we are doomed to suffer the slowness due to lockdep checks at the same time.
Sure we can continue to support having both, but I think we should also think about a solution to handle lock events without it in the future. That will require some minimal lockdep functionalities (keeping the lockdep map, and class hashes).
Thanks.
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