linux-kernel - Re: [PATCH v2 tip/perf/core 1/6] perf symbols: find symbols in different mount namespace

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Date:   Fri, 7 Jul 2017 12:36:41 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Krister Johansen <kjlx@...pleofstupid.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Thomas-Mich Richter <tmricht@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 tip/perf/core 1/6] perf symbols: find symbols in
 different mount namespace

On Thu, Jul 06, 2017 at 04:41:30PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Jul 05, 2017 at 06:48:08PM -0700, Krister Johansen escreveu:
> > Teach perf how to resolve symbols from binaries that are in a different
> > mount namespace from the tool.  This allows perf to generate meaningful
> > stack traces even if the binary resides in a different mount namespace
> > from the tool.
> 
> I was trying to find a way to test after applying each of the patches in
> this series, when it ocurred to me that if a process that appears on a
> perf.data file has exit, how can we access /proc/%ITS_PID/something?

You're correct.  We can't access /proc/<PID>/whatever once the process
has exited.  That was the impeteus for patches 4 and 6, which allow us
to capture the binary (and debuginfo, if it exists) into the buildid
cache so that if we do have a trace that exists after a process or
container exists, we'll still be able to resolve some of the symbols.

-K