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

From: Masami Hiramatsu
Date: Thu Oct 24 2019 - 11:36:55 EST


Hi Arnaldo,

On Thu, 24 Oct 2019 10:43:25 -0300
Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> 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@/home/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@/home/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@/home/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@xxxxxxxxxx>