linux-kernel - Re: [BUGFIX PATCH 0/3] perf/probe: Fix available line bugs

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Date:   Fri, 25 Oct 2019 00:36:48 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX PATCH 0/3] perf/probe: Fix available line bugs

Hi Arnaldo,

On Thu, 24 Oct 2019 10:43:25 -0300
Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> Em Thu, Oct 24, 2019 at 06:12:27PM +0900, Masami Hiramatsu escreveu:
> > Hi,
> > 
> > Here are some bugfixes related to showing available line (--line/-L)
> > option. I found that gcc generated some subprogram DIE with only
> > range attribute but no entry_pc or low_pc attributes.
> > In that case, perf probe failed to show the available lines in that
> > subprogram (function). To fix that, I introduced some bugfixes to
> > handle such cases correctly.
> 
> Thanks, applied, next time please provide concrete examples for things
> that don't work before and gets fixed with your patches, this way we can
> more easily reproduce your steps.

OK, I'll try, but I found that this kind of examples depend on
gcc optimization (like build option, gcc version etc.) So even if you
can not reproduce, don't be disappointed ;)

Anyway, for this series, I found that clear_tasks_mm_cpumask() caused
the (triple) issues.

So, without any patch, I got below error.

$ tools/perf/perf probe -k ../build-x86_64/vmlinux -L clear_tasks_mm_cpumask
Specified source line is not found.
  Error: Failed to show lines.

Hmm, there should be something wrong... so I fixed it with [1/3].
(to find appropriate target function, you can use "eu-readelf -w vmlinux"
 and find the subprogram which has no entry_pc nor low_pc but has ranges)

$ tools/perf/perf probe -k ../build-x86_64/vmlinux -L clear_tasks_mm_cpumask
<clear_tasks_mm_cpumask@...me/mhiramat/ksrc/mincs/work/linux/linux/kernel/cpu.c:
         void clear_tasks_mm_cpumask(int cpu)
      1  {
      2         struct task_struct *p;
         
                /*
                 * This function is called after the cpu is taken down and marke
                 * offline, so its not like new tasks will ever get this cpu set
                 * their mm mask. -- Peter Zijlstra
                 * Thus, we may use rcu_read_lock() here, instead of grabbing
                 * full-fledged tasklist_lock.
                 */
     11         WARN_ON(cpu_online(cpu));
     12         rcu_read_lock();
     13         for_each_process(p) {
     14                 struct task_struct *t;
         
                        /*
                         * Main thread might exit, but other threads may still h
                         * a valid mm. Find one.
                         */
     20                 t = find_lock_task_mm(p);
     21                 if (!t)
                                continue;
                        cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
                        task_unlock(t);
                }
     26         rcu_read_unlock();
     27  }
         
OK! it looks working... but a bit wired. why line #0, #23 and #24 are not
available?
I investigated it and found a wrong logic and fixed it with [2/3]

$ tools/perf/perf probe -k ../build-x86_64/vmlinux -L clear_tasks_mm_cpumask:20
<clear_tasks_mm_cpumask@...me/mhiramat/ksrc/mincs/work/linux/linux/kernel/cpu.c:
     20                 t = find_lock_task_mm(p);
     21                 if (!t)
                                continue;
     23                 cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
     24                 task_unlock(t);
                }
     26         rcu_read_unlock();
     27  }

OK, it found line #23 and #24. And add [3/3]

$ tools/perf/perf probe -k ../build-x86_64/vmlinux -L clear_tasks_mm_cpumask
<clear_tasks_mm_cpumask@...me/mhiramat/ksrc/mincs/work/linux/linux/kernel/cpu.c:
      0  void clear_tasks_mm_cpumask(int cpu)
      1  {
      2         struct task_struct *p;

OK, so now it works well :)

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>