linux-kernel - Re: [RFC] perf/core: allow ftrace for functions in kernel/event/core.c

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Date:   Tue, 26 May 2020 23:54:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>
Subject: Re: [RFC] perf/core: allow ftrace for functions in
 kernel/event/core.c

On Tue, May 26, 2020 at 09:46:29PM +0000, Song Liu wrote:
> 
> 
> > On May 26, 2020, at 2:39 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Tue, May 26, 2020 at 02:28:26PM -0700, Song Liu wrote:
> >> It is useful to trace functions in kernel/event/core.c. Allow ftrace for
> >> them by removing $(CC_FLAGS_FTRACE) from Makefile.
> > 
> > Did you try using the ftrace event with perf with this on?
> 
> I have tried a few things, like 
> 
>   perf stat -e probe:perf_read -I 1000
>   perf record -e probe:__x64_sys_perf_event_open -aR
> 
> They all work fine. 
> 
> Do you have some tricky functions that we should double check?

I've no idea what probe: does. iirc there's something like
ftrace:function that is like regular function tracing.

At some point using that made the kernel really sick due to recursion
between ftrace and perf. Quite possibly that's been fixed, dunno.