Messages in this thread | | | Date | Thu, 6 Apr 2017 14:48:03 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH 3/4] tracing: Add stack_tracer_disable/enable() functions |
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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 > > > >
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