linux-kernel - Re: [PATCH v2] perf report: fix off-by-one for non-activation frames

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Date:   Tue, 16 May 2017 18:17:26 +0200
From:   Milian Wolff <milian.wolff@...b.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Yao Jin <yao.jin@...ux.intel.com>, kernel-team@....com
Subject: Re: [PATCH v2] perf report: fix off-by-one for non-activation frames

On Dienstag, 16. Mai 2017 16:38:29 CEST Namhyung Kim wrote:
> On Tue, May 16, 2017 at 10:59:51AM +0200, Milian Wolff wrote:
> > As the documentation for dwfl_frame_pc says, frames that
> > are no activation frames need to have their program counter
> > decremented by one to properly find the function of the caller.
> > 
> > This fixes many cases where perf report currently attributes
> > the cost to the next line. I.e. I have code like this:
> > 
> > ~~~~~~~~~~~~~~~
> > 
> >   #include <thread>
> >   #include <chrono>
> >   
> >   using namespace std;
> >   
> >   int main()
> >   {
> >   
> >     this_thread::sleep_for(chrono::milliseconds(1000));
> >     this_thread::sleep_for(chrono::milliseconds(100));
> >     this_thread::sleep_for(chrono::milliseconds(10));
> >     
> >     return 0;
> >   
> >   }
> > 
> > ~~~~~~~~~~~~~~~
> 
> It'd be nice if the test program has a signal frame for verification.

I have pretty much zero experience about signals. Would it be enough to add a 
signal handler for, say, SIGUSR1 to my test application and then trigger a 
sleep when that signal is delivered? If that should be enough, I'll write and 
test it out.

<snip>

> > diff --git a/tools/perf/util/unwind-libunwind-local.c
> > b/tools/perf/util/unwind-libunwind-local.c index
> > f8455bed6e65..30ab26375c80 100644
> > --- a/tools/perf/util/unwind-libunwind-local.c
> > +++ b/tools/perf/util/unwind-libunwind-local.c
> > @@ -690,8 +690,22 @@ static int get_entries(struct unwind_info *ui,
> > unwind_entry_cb_t cb,> 
> >  		if (ret)
> >  		
> >  			display_error(ret);
> > 
> > +		bool previous_frame_was_signal = false;
> > 
> >  		while (!ret && (unw_step(&c) > 0) && i < max_stack) {
> >  		
> >  			unw_get_reg(&c, UNW_REG_IP, &ips[i]);
> > 
> > +
> > +			/*
> > +			 * Decrement the IP for any non-activation frames.
> > +			 * this is required to properly find the srcline
> > +			 * for caller frames.
> > +			 * See also the documentation for dwfl_frame_pc,
> > +			 * which this code tries to replicate.
> > +			 */
> > +			bool frame_is_signal = unw_is_signal_frame(&c) > 0;
> > +			if (!previous_frame_was_signal && !frame_is_signal)
> > +				--ips[i];
> > +			previous_frame_was_signal = frame_is_signal;
> 
> Does it need to check previous frame too?

That's what dwfl_frame_pc does, if I'm not misunderstanding it's source code?

Bye

-- 
Milian Wolff | milian.wolff@...b.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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