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

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Date:   Wed, 27 May 2020 18:40:33 +0000
From:   Song Liu <songliubraving@...com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 10:40 PM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> On Tue, 26 May 2020 21:46:29 +0000
> Song Liu <songliubraving@...com> 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. 
> 
> Did you try using perf with function-tracer or function-graph tracer?
> If you just want to trace those functions with kprobes, you can 
> build your kernel with CONFIG_KPROBE_EVENTS_ON_NOTRACE=y, which allows
> you to probe perf_read etc.

Thanks for the hint. 

kprobe does work with CONFIG_KPROBE_EVENTS_ON_NOTRACE, but BPF trampoline
doesn't work. If it is safe, removing $(CC_FLAGS_FTRACE) is preferable.

Thanks,
Song