linux-kernel - Re: [PATCH] perf tools: Fix not finding kcore in buildid cache

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Date:	Wed, 27 Nov 2013 11:42:31 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf tools: Fix not finding kcore in buildid cache

Em Wed, Nov 27, 2013 at 12:52:47PM +0100, Ingo Molnar escreveu:
> 
> * Adrian Hunter <adrian.hunter@...el.com> wrote:
> 
> > On 27/11/13 13:32, Ingo Molnar wrote:
> > > 
> > > * Adrian Hunter <adrian.hunter@...el.com> wrote:
> > > 
> > >> On 26/11/13 18:15, Ingo Molnar wrote:
> > >>>
> > >>> * Adrian Hunter <adrian.hunter@...el.com> wrote:
> > >>>
> > >>>> The logic was not looking in the buildid cache for kcore
> > >>>> if the host kernel buildid did not match the recorded
> > >>>> kernel buildid.
> > >>>
> > >>> It would be nice to also include the specific failure mode in the 
> > >>> changelog: exactly how can users run into this. That will be more 
> > >>> useful to most people reading these changelogs than a more abstract 
> > >>> bug description.
> > >>
> > >> The preconditions for the problem are that kcore is in the buildid
> > >> cache and that the running kernel has a different buildid e.g.
> > >>
> > >> $ perf buildid-list | grep kernel.kallsyms
> > >> b18fbe769dea12513028a4287dae02efc1582c29 [kernel.kallsyms]
> > >>
> > >> $ perf record -o /tmp/junk true >/dev/null 2>&1 ; perf buildid-list -i /tmp/junk | grep kernel.kallsyms
> > >> 504f9fd9328c40714995ce76fe1354c6e90136ce [kernel.kallsyms]
> > >>
> > >>
> > >>
> > >> Before the patch:
> > >>
> > >>
> > >> $ perf script -v >out.txt
> > >> build id event received for [kernel.kallsyms]: b18fbe769dea12513028a4287dae02efc1582c29
> > >> <SNIP>
> > >> Using /home/ahunter/.debug/[kernel.kallsyms]/b18fbe769dea12513028a4287dae02efc1582c29 for symbols
> > >>
> > >> perf script stopped
> > >>
> > >>
> > >> After the patch:
> > >>
> > >>
> > >> $ perf script -v >out.txt
> > >> build id event received for [kernel.kallsyms]: b18fbe769dea12513028a4287dae02efc1582c29
> > >> <SNIP>
> > >> Using /home/ahunter/.debug/[kernel.kcore]/b18fbe769dea12513028a4287dae02efc1582c29/2013112617084061/kcore for kernel object code
> > >> Using /home/ahunter/.debug/[kernel.kcore]/b18fbe769dea12513028a4287dae02efc1582c29/2013112617084061/kallsyms for symbols
> > >>
> > >> perf script stopped
> > > 
> > > So the result of the patch is that kernel symbols get 
> > > resolved/annotated correctly, as expected from the live kcore feature, 
> > > right?
> > 
> > Yes but this is the non-live case i.e. the kernel has changed and we are
> > looking at a special copy of kcore that we placed in the buildid cache
> > (using "perf buildid-cache -v -k /proc/kcore") when the data was recorded.
> 
> Okay.

Thanks for the questions and answers, I'll edit it and add to the patch.

- Arnaldo
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