linux-kernel - Re: [PATCH 10/11] tracing/perf: Fix lock events recursions in the fast path

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Date:	Sun, 28 Feb 2010 23:24:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Lai Jiangshan <laijs@...fujitsu.com>,
	paulmck@...ux.vnet.ibm.com, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Mackerras <paulus@...ba.org>,
	Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
	Li Zefan <lizf@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 10/11] tracing/perf: Fix lock events recursions in the
	fast path

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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