Re: [PATCH 1/2] ftrace: print stack usage right before Oops

[Minchan Kim <minchan@kernel.org>]

On Wed, May 28, 2014 at 12:18:32PM -0400, Steven Rostedt wrote:
> On Wed, 28 May 2014 15:53:58 +0900
> Minchan Kim <minchan@kernel.org> wrote:
> 
> > While I played with my own feature(ex, something on the way to reclaim),
> > kernel went to oops easily. I guessed reason would be stack overflow
> > and wanted to prove it.
> > 
> > I found stack tracer which would be very useful for me but kernel went
> > oops before my user program gather the information via
> > "watch cat /sys/kernel/debug/tracing/stack_trace" so I couldn't get an
> > stack usage of each functions.
> > 
> > What I want was that emit the kernel stack usage when kernel goes oops.
> > 
> > This patch records callstack of max stack usage into ftrace buffer
> > right before Oops and print that information with ftrace_dump_on_oops.
> > At last, I can find a culprit. :)
> > 
> 
> This is not dependent on patch 2/2, nor is 2/2 dependent on this patch,
> I'll review this as if 2/2 does not exist.

Yeb, Thanks!

> 
> 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  kernel/trace/trace_stack.c | 32 ++++++++++++++++++++++++++++++--
> >  1 file changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> > index 5aa9a5b9b6e2..5eb88e60bc5e 100644
> > --- a/kernel/trace/trace_stack.c
> > +++ b/kernel/trace/trace_stack.c
> > @@ -51,6 +51,30 @@ static DEFINE_MUTEX(stack_sysctl_mutex);
> >  int stack_tracer_enabled;
> >  static int last_stack_tracer_enabled;
> >  
> > +static inline void print_max_stack(void)
> > +{
> > +	long i;
> > +	int size;
> > +
> > +	trace_printk("        Depth    Size   Location"
> > +			   "    (%d entries)\n"
> 
> Please do not break strings just to satisfy that silly 80 character
> limit. Even Linus Torvalds said that's pretty stupid.

I just copied existing code from trace_stack.c.
Okay, I will fix. :)

> 
> Also, do not use trace_printk(). It is not made to be included in a
> production kernel. It reserves special buffers to make it as fast as
> possible, and those buffers should not be created in production
> systems. In fact, I will probably add for 3.16 a big warning message
> when trace_printk() is used.
> 
> Since this is a bug, why not just use printk() instead?

Thanks for the info. I will use printk(KERN_EMERG).

> 
> BTW, wouldn't this this function crash as well if the stack is already
> bad?

It wasn't crashed until code has data from corrupted threadinfo.

> 
> -- Steve
> 
> > +			   "        -----    ----   --------\n",
> > +			   max_stack_trace.nr_entries - 1);
> > +
> > +	for (i = 0; i < max_stack_trace.nr_entries; i++) {
> > +		if (stack_dump_trace[i] == ULONG_MAX)
> > +			break;
> > +		if (i+1 == max_stack_trace.nr_entries ||
> > +				stack_dump_trace[i+1] == ULONG_MAX)
> > +			size = stack_dump_index[i];
> > +		else
> > +			size = stack_dump_index[i] - stack_dump_index[i+1];
> > +
> > +		trace_printk("%3ld) %8d   %5d   %pS\n", i, stack_dump_index[i],
> > +				size, (void *)stack_dump_trace[i]);
> > +	}
> > +}
> > +
> >  static inline void
> >  check_stack(unsigned long ip, unsigned long *stack)
> >  {
> > @@ -149,8 +173,12 @@ check_stack(unsigned long ip, unsigned long *stack)
> >  			i++;
> >  	}
> >  
> > -	BUG_ON(current != &init_task &&
> > -		*(end_of_stack(current)) != STACK_END_MAGIC);
> > +	if ((current != &init_task &&
> > +		*(end_of_stack(current)) != STACK_END_MAGIC)) {
> > +		print_max_stack();
> > +		BUG();
> > +	}
> > +
> >   out:
> >  	arch_spin_unlock(&max_stack_lock);
> >  	local_irq_restore(flags);
> 
> --
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-- 
Kind regards,
Minchan Kim

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