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Date: Thu, 15 Jun 2017 21:20:53 +0200
From: Jan Kratochvil <jan.kratochvil@redhat.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>,
        Milian Wolff <milian.wolff@kdab.com>, Linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org, Namhyung Kim <namhyung@kernel.org>,
        acme@kernel.org
Subject: Re: [PATCH 2/2] perf report: report module before querying
 isactivation in dwfl unwind
Message-ID: <20170615192053.GA4615@host1.jankratochvil.net>
References: <20170602143753.16907-1-milian.wolff@kdab.com>
 <20170602143753.16907-2-milian.wolff@kdab.com>
 <20170602152508.GC7901@redhat.com>
 <20170605104720.GC28871@krava>
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On Mon, 05 Jun 2017 12:47:20 +0200, Jiri Olsa wrote:
> On Fri, Jun 02, 2017 at 12:25:08PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Jun 02, 2017 at 04:37:53PM +0200, Milian Wolff escreveu:
> > > The PC returned by dwfl_frame_pc may map into a not-yet-reported
> > > module. We have to report it before we continue unwinding. But when
> > > we query for the isactivation flag in dwfl_frame_pc, libdw will
> > > actually do one more unwinding step internally which can then break
> > > and lead to missed frames or broken stacks.
> > > 
> > > With libunwind we get e.g.:
> 
> sounds like a libdw design issue.. Jan is there a specific way
> to handle the case that Milian described?

I agree with the patch.

Just be aware for performance then the __report_module() call in entry() is no
longer useful as the '--pc' adjustment should never cross a module boundary.

Although for perf unwinding performance there is still a wide gap there
(such as caching the loaded modules for multiple backtraces.)


Thanks,
Jan