Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sun, 28 Feb 2010 23:24:16 +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.
Actually, looking at lock_acquire/release/acquired/contended, they are all performing their job under a raw_lock_irq_save() window, so it doesn't seem we are losing anything.
Something that could be nice though: dropping this irq saving from the main window, add the perf style recursion protection, and eventually have a raw_local_irq_save only when we take internal lockdep locks. This will let irqs happen during lockdep checks.
I just guess the irq disabled thing is also protecting something else, something I'll probably discover after testing that :)
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