Re: perf probe: Trying to read a 'ctx' named local variable

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 17 Jul 2023 15:06:10 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> Hi Masami,
> 
> 	Trying to use perf to debug perf I stumbled on this:
> 
> [root@quaco ~]# perf probe -x ~/bin/perf -L sigtrap_handler
> <sigtrap_handler@/home/acme/git/perf-tools/tools/perf/tests/sigtrap.c:0>
>       0  sigtrap_handler(int signum __maybe_unused, siginfo_t *info, void *ucontext __maybe_unused)
>          {
>                 if (!__atomic_fetch_add(&ctx.signal_count, 1, __ATOMIC_RELAXED))
>       3                 ctx.first_siginfo = *info;
>       4         __atomic_fetch_sub(&ctx.tids_want_signal, syscall(SYS_gettid), __ATOMIC_RELAXED);
>          }
> 
>          static void *test_thread(void *arg)
> 
> [root@quaco ~]# perf probe -x ~/bin/perf sigtrap_handler:4 "ctx.signal_count"
> Failed to write event: Invalid argument
> Please upgrade your kernel to at least 3.14 to have access to feature @ctx
>   Error: Failed to add events.
> [root@quaco ~]#

Ah, this is a wrong usage (I'm also trapped the same thing). Correctly, it should
pass it as a single parameter.

perf probe -x ~/bin/perf "sigtrap_handler:4 ctx.signal_count"

This is for adding several events at the same time. But now it becomes just a trap.
Can I change the syntax from

perf probe [OPTIONS] <PROBE-DEFINITION> [PROBE-DEFINITION...]

To,

perf probe [OPTIONS] <PROBE-POINT> [ARGS...]

?

We can keep --add having optional parameter as below.

perf probe [OPTIONS] --add <PROBE-DEFINITION>

Thank you,

> 
> [root@quaco ~]# uname -a
> Linux quaco 6.3.8-100.fc37.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Jun 15 01:51:54 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
> [root@quaco ~]# rpm -qa | grep kernel-debuginfo
> kernel-debuginfo-common-x86_64-6.3.8-100.fc37.x86_64
> kernel-debuginfo-6.3.8-100.fc37.x86_64
> [root@quaco ~]#
> 
> -- 
> 
> - Arnaldo


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>