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

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Date:   Tue, 26 May 2020 22:08:37 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
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 May 26, 2020, at 2:54 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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.

Those are kprobes. I have CONFIG_HAVE_KPROBES_ON_FTRACE=y in .config. 

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

I also tried on ctx_sched_out() and event_sched_out(). They also work
as expected. 

Thanks,
Song