linux-kernel - Re: [PATCH] perf c2c report: Fix crash for empty browser

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Message-ID: <c5cb64016f7da6c2abdef4a05ea74425@autistici.org>
Date:   Thu, 26 Jul 2018 23:31:34 +0000
From:   rodia@...istici.org
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf c2c report: Fix crash for empty browser

On 2018-07-26 19:30, Arnaldo Carvalho de Melo wrote:
> Em Tue, Jul 24, 2018 at 08:20:08AM +0200, Jiri Olsa escreveu:
>> Do not try to display entry details if there's
>> not any. Currently this ends up in crash:
>>   $ perf c2c report
>>   perf: Segmentation fault
> 
> How to replicate this?
> 
> I tried:
> 
> $ perf record sleep 1
> $ perf c2c report
> 
> But it didn't segfault

Similarly I have tried :
$ perf record sleep 1
$ perf c2c report
Then Press `d` to show the cache-line contents.
This replies the segfault on my machine (4.17.8-1).
The patch mentioned above should solve it, even tough I am not sure as I 
haven't been able to recompile the kernel.

The segfault by itself seems to be due to the report logic, as it did 
not expect to report on an empty browser.
What has stepped me back is that application which I have been testing 
with rely on multiple threads instantiated through pthread, which should 
be counted in user-level threads right? But they still seem to return an 
empty browser.

When instead c2c is runned system-wide, with an application running on 
multiple threads like firefox or julia, cache hits are measured and also 
they are traced back in the source code.

Anyway, thanks for your support!