linux-kernel - Re: x86_pmu_start WARN

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Date:	Thu, 20 Feb 2014 19:15:38 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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 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.

> > 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.
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