Re: [PATCH 3/4] tracing: Add stack_tracer_disable/enable() functions

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 6 Apr 2017 11:12:22 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Thu, Apr 06, 2017 at 12:42:40PM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > 
> > There are certain parts of the kernel that can not let stack tracing
> > proceed (namely in RCU), because the stack tracer uses RCU, and parts of RCU
> > internals can not handle having RCU read side locks taken.
> > 
> > Add stack_tracer_disable() and stack_tracer_enable() functions to let RCU
> > stop stack tracing on the current CPU as it is in those critical sections.  
> 
> s/as it is in/when it is in/?
> 
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>  
> 
> One quibble above, one objection below.
> 
> 							Thanx, Paul
> 
> > ---
> >  include/linux/ftrace.h     |  6 ++++++
> >  kernel/trace/trace_stack.c | 28 ++++++++++++++++++++++++++++
> >  2 files changed, 34 insertions(+)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index ef7123219f14..40afee35565a 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -286,6 +286,12 @@ int
> >  stack_trace_sysctl(struct ctl_table *table, int write,
> >  		   void __user *buffer, size_t *lenp,
> >  		   loff_t *ppos);
> > +
> > +void stack_tracer_disable(void);
> > +void stack_tracer_enable(void);
> > +#else
> > +static inline void stack_tracer_disable(void) { }
> > +static inline void stack_tracer_enabe(void) { }
> >  #endif
> > 
> >  struct ftrace_func_command {
> > diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> > index 05ad2b86461e..5adbb73ec2ec 100644
> > --- a/kernel/trace/trace_stack.c
> > +++ b/kernel/trace/trace_stack.c
> > @@ -41,6 +41,34 @@ static DEFINE_MUTEX(stack_sysctl_mutex);
> >  int stack_tracer_enabled;
> >  static int last_stack_tracer_enabled;
> > 
> > +/**
> > + * stack_tracer_disable - temporarily disable the stack tracer
> > + *
> > + * There's a few locations (namely in RCU) where stack tracing
> > + * can not be executed. This function is used to disable stack
> > + * tracing during those critical sections.
> > + *
> > + * This function will disable preemption. stack_tracer_enable()
> > + * must be called shortly after this is called.
> > + */
> > +void stack_tracer_disable(void)
> > +{
> > +	preempt_disable_notrace();  
> 
> Interrupts are disabled in all current call points, so you don't really
> need to disable preemption.  I would normally not worry, given the
> ease-of-use improvements, but some people get annoyed about even slight
> increases in idle-entry overhead.

My worry is that we add another caller that doesn't disable interrupts
or preemption.

I could add a __stack_trace_disable() that skips the disabling of
preemption, as the "__" usually denotes the call is "special".

-- Steve

> 
> > +	this_cpu_inc(trace_active);
> > +}
> > +
> > +/**
> > + * stack_tracer_enable - re-enable the stack tracer
> > + *
> > + * After stack_tracer_disable() is called, stack_tracer_enable()
> > + * must shortly be called afterward.
> > + */
> > +void stack_tracer_enable(void)
> > +{
> > +	this_cpu_dec(trace_active);
> > +	preempt_enable_notrace();  
> 
> Ditto...
> 
> > +}
> > +
> >  void stack_trace_print(void)
> >  {
> >  	long i;
> > -- 
> > 2.10.2
> > 
> >