linux-kernel - Re: x86_pmu_start WARN

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Message-ID: <20140220182332.GE22728@two.firstfloor.org>
Date:	Thu, 20 Feb 2014 19:23:32 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: x86_pmu_start WARN_ON.

On Thu, Feb 20, 2014 at 07:15:38PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 20, 2014 at 09:31:19AM -0800, Andi Kleen wrote:
> > Peter Zijlstra <peterz@...radead.org> writes:
> > >
> > > It will; trace_printk() works without -pg, I think you didn't read the
> > > instructions very well.
> > 
> > Ok, you enable and disable it again.  I won't guess why you do that.
> 
> To grow the trace buffers; it starts with just a few pages per cpu; once
> you switch to an actual tracer it allocates a sensible amount.
> 
> You can grow it with another interface; but then I'd have to like
> remember what that was and how big the normal buffers are. Simply
> toggling between tracers is far easier.

I see.

> 
> > > And there's a very good reason not to apply your patch; you can route
> > > the function tracer into perf, guess what happens when perf calls the
> > > function tracer again :-)
> > 
> > How? 
> 
> I think by using the /debug/tracing/events/ftrace/function event, but
> I'm not actually sure, I've never used it nor did I write the code to do
> it. Jolsa did all that IIRC.
> 
> All I know is that we had some 'fun' bugs around there sometime back.

Ok.

I don't think it would be a problem in any case, the ftrace code has recursion
protection.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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