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

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Message-ID: <20200526180437.45aed950@gandalf.local.home>
Date:   Tue, 26 May 2020 18:04:37 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Song Liu <songliubraving@...com>,
        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

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.

-- Steve