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From:   Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Date:   Tue, 11 Feb 2020 10:46:24 -0300
To:     Marek Majkowski <marek@cloudflare.com>
Cc:     Jiri Olsa <jolsa@redhat.com>, Ivan Babrou <ivan@cloudflare.com>,
        kernel-team <kernel-team@cloudflare.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>, sashal@kernel.org,
        Kenton Varda <kenton@cloudflare.com>
Subject: Re: perf not picking up symbols for namespaced processes
Message-ID: <20200211134624.GA32066@kernel.org>
References: <CABWYdi1ZKR=jmKnjoJTik08Q9uJBvyZ4W0C29iPiUJ5ef1obvw@mail.gmail.com>
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Em Tue, Feb 11, 2020 at 10:06:35AM +0000, Marek Majkowski escreveu:
> Jirka,
> 
> On Tue, Feb 4, 2020 at 7:27 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > > 11913 openat(AT_FDCWD, "/proc/9512/ns/mnt", O_RDONLY) = 197
> > > 11913 setns(197, CLONE_NEWNS) = 0
> > > 11913 stat("/home/marek/bin/runsc-debug", 0x7fffffff8480) = -1 ENOENT
> > > (No such file or directory)
> > > 11913 setns(196, CLONE_NEWNS) = 0
> >
> > hi,
> > could you guys please share more details on what you run exactly,
> > and perhaps that change you mentioned?
> 
> I was debugging gvisor (runsc), which does execve(/proc/self/exe), and
> then messes up with its mount namespace. The effect is that the binary
> running doesn't exist in the mount namespace. This confuses perf,
> which fails to load symbols for that process.
> 
> To my understanding, by default, perf looks for the binary in the
> process mount namespace. In this case clearly the binary wasn't there.
> Ivan wrote a rough patch [1], which I just confirmed works. The patch
> attempts, after a failure to load binary from pids mount namespace, to
> load binary from the default mount namespace (the one running perf).
> 
> [1] https://lkml.org/lkml/2019/12/5/878

That is a fallback that works in this specific case, and, with a warning
or some explicitely specified option makes perf work with this specific
usecase, but either a warning ("fallback to root namespace binary
/foo/bar") or the explicit option, please, is that what that patch does?

- Arnaldo