linux-kernel - Re: [RFC][PATCH 06/10] tracing: Add trigger file for trace

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Date:   Mon, 14 May 2018 11:04:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Clark Williams <williams@...hat.com>,
        Karim Yaghmour <karim.yaghmour@...rsys.com>,
        Brendan Gregg <bgregg@...flix.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [RFC][PATCH 06/10] tracing: Add trigger file for trace_markers
 tracefs/ftrace/print

On Mon, 14 May 2018 12:13:22 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> On Fri, May 11, 2018 at 03:49:33PM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > 
> > Allow writing to the trace_markers file initiate triggers defined in
> > tracefs/ftrace/print/trigger file. This will allow of user space to trigger
> > the same type of triggers (including histograms) that the trace events use.
> > 
> > Cc: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > Cc: Clark Williams <williams@...hat.com>
> > Cc: Karim Yaghmour <karim.yaghmour@...rsys.com>
> > Cc: Brendan Gregg <bgregg@...flix.com>
> > Suggested-by: Joel Fernandes <joelaf@...gle.com>
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > ---  
> 
> [SNIP]
> > diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
> > index e3a658bac10f..de3fce14cd00 100644
> > --- a/kernel/trace/trace_entries.h
> > +++ b/kernel/trace/trace_entries.h
> > @@ -230,7 +230,7 @@ FTRACE_ENTRY(bprint, bprint_entry,
> >  	FILTER_OTHER
> >  );
> >  
> > -FTRACE_ENTRY(print, print_entry,
> > +FTRACE_ENTRY_REG(print, print_entry,
> >  
> >  	TRACE_PRINT,
> >  
> > @@ -242,7 +242,9 @@ FTRACE_ENTRY(print, print_entry,
> >  	F_printk("%ps: %s",
> >  		 (void *)__entry->ip, __entry->buf),
> >  
> > -	FILTER_OTHER
> > +	FILTER_OTHER,
> > +
> > +	 ftrace_event_register  
> 
> I wonder this is still needed since you added __find_event_file()
> which ignores the reg field.  Maybe I'm missing something..

No, that's not the reason for it. The reason is due to the
event_trigger_callback() function. I'll update the changelog to specify
this.

We have something like:

 # echo snapshot > events/ftrace/print/trigger

Which will do:

  event_trigger_write() {
    event_trigger_regex_write() {
      trigger_process_regex() {
        for p in trigger_commands {
          p->func(); /* trigger_snapshot_cmd->func */
            event_trigger_callback() {
              cmd_ops->reg() /* register_trigger() */ {
                trace_event_trigger_enable_disable() {
                  trace_event_enable_disable() {
                    call->class->reg();

Without this patch, we jump to NULL and BOOM!
                

> 
> Anyway it looks whitespace damaged.
>

Thanks, I'll fix it up.

-- Steve