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

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Message-ID: <20100206111209.GC5062@nowhere>
Date:	Sat, 6 Feb 2010 12:12:12 +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.


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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