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

[Song Liu]

> On May 26, 2020, at 2:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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