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

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Message-ID: <595AB855-8E84-4367-AEEC-C140F035C9B4@fb.com>
Date:   Fri, 5 Jun 2020 21:58:48 +0000
From:   Song Liu <songliubraving@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Ingo Molnar <mingo@...hat.com>,
        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

Hi Steven,

> On May 26, 2020, at 3:04 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Tue, 26 May 2020 23:54:15 +0200
> 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.
>> 
>> At some point using that made the kernel really sick due to recursion
>> between ftrace and perf. Quite possibly that's been fixed, dunno.
> 
> In the early days there was a lot of issues with recursions, but I added a
> lot of recursion protection since then. I'll give this patch a spin and see
> if I can make it crash.

How does this work in your tests? 

Thanks,
Song