linux-kernel - Re: [PATCH][GIT PULL] tracing: add trace

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Date:	Tue, 19 May 2009 21:14:13 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH][GIT PULL] tracing: add trace_stack interface


On Wed, 20 May 2009, Frederic Weisbecker wrote:

> On Tue, May 19, 2009 at 08:44:50PM -0400, Steven Rostedt wrote:
> > 
> > Ingo,
> > 
> > Please pull the latest tip/tracing/ftrace tree, which can be found at:
> > 
> >   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> > tip/tracing/ftrace
> > 
> > 
> > Steven Rostedt (1):
> >       tracing: add trace_stack interface
> > 
> > ----
> >  include/linux/kernel.h |    2 ++
> >  kernel/trace/trace.c   |   15 +++++++++++++++
> >  2 files changed, 17 insertions(+), 0 deletions(-)
> > ---------------------------
> > commit b85d1029ba534376d203c3ac83a2e5a4d6876cb6
> > Author: Steven Rostedt <srostedt@...hat.com>
> > Date:   Tue May 19 18:54:16 2009 -0400
> > 
> >     tracing: add trace_stack interface
> >     
> >     This patch adds the global function
> >     
> >       void trace_stack(void)
> >     
> >     This allows a developer to find where a function is called. For example,
> >     if you want to know who calls __netif_reschedule, you can add
> >     
> >      static inline void __netif_reschedule(struct Qdisc *q)
> >      {
> >      	struct softnet_data *sd;
> >      	unsigned long flags;
> >     
> >     +	trace_stack();
> >      	local_irq_save(flags);
> >      	sd = &__get_cpu_var(softnet_data);
> >      	q->next_sched = sd->output_queue;
> >     
> >     And see the following in the trace output:
> >     
> >      <= __ftrace_trace_stack
> >      <= trace_stack
> 
> 
> 
> I guess the depth should begin here :-)

Yeah, I thought about that but since it only prints out the stack trace, 
this is sorta the "this is a stack trace" header.
> 
> 
> 
> >      <= __netif_schedule
> >      <= dev_watchdog
> >      <= run_timer_softirq
> >      <= __do_softirq
> >      <= call_softirq
> >      <= do_softirq
> >      <= irq_exit
> >     
> >     Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 883cd44..347a0f1 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -511,6 +511,7 @@ extern int
> >  __ftrace_vprintk(unsigned long ip, const char *fmt, va_list ap);
> >  
> >  extern void ftrace_dump(void);
> > +extern void trace_stack(void);
> >  #else
> >  static inline void
> >  ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3) { }
> > @@ -531,6 +532,7 @@ ftrace_vprintk(const char *fmt, va_list ap)
> >  	return 0;
> >  }
> >  static inline void ftrace_dump(void) { }
> > +static inline void trace_stack(void) { }
> >  #endif /* CONFIG_TRACING */
> >  
> >  /*
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index dd40d23..33ab502 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -1046,6 +1046,21 @@ void __trace_stack(struct trace_array *tr,
> >  	__ftrace_trace_stack(tr, flags, skip, pc);
> >  }
> >  
> > +/**
> > + * trace_stack - dump the stack into the trace buffer
> > + *
> > + * Use this to record the stack trace into the ring buffer. If you
> > + * want to know a caller of a function, place a call to trace_stack
> > + * and the ftrace trace file will display who the callers were.
> > + */
> > +void trace_stack(void)
> > +{
> > +	unsigned long flags;
> > +
> > +	local_save_flags(flags);
> > +	__trace_stack(&global_trace, flags, 0, preempt_count());
> > +}
> 
> 
> 
> Stupidly I use a very similar patch while working on reiserfs
> but I forgot to post it.
> 
> Another trick is to use trace_printk() combined with:
> 
> echo stacktrace > trace_option

Yeah, but when you are debugging development work, doing that at every 
boot up becomes a pain. You can add it to a rc script but still.

I wrote this because of the development I'm doing in the interrupt 
threads. I was using this a lot to find what is calling what.

-- Steve

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