Re: [patch V3 21/29] tracing: Use percpu stack trace buffer more intelligently

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Apr 25, 2019 at 11:45:14AM +0200, Thomas Gleixner wrote:
> @@ -2788,29 +2798,32 @@ static void __ftrace_trace_stack(struct
>  	 */
>  	preempt_disable_notrace();
>  
> -	use_stack = __this_cpu_inc_return(ftrace_stack_reserve);
> +	stackidx = __this_cpu_inc_return(ftrace_stack_reserve);
> +
> +	/* This should never happen. If it does, yell once and skip */
> +	if (WARN_ON_ONCE(stackidx >= FTRACE_KSTACK_NESTING))
> +		goto out;
> +
>  	/*
> -	 * We don't need any atomic variables, just a barrier.
> -	 * If an interrupt comes in, we don't care, because it would
> -	 * have exited and put the counter back to what we want.
> -	 * We just need a barrier to keep gcc from moving things
> -	 * around.
> +	 * The above __this_cpu_inc_return() is 'atomic' cpu local. An
> +	 * interrupt will either see the value pre increment or post
> +	 * increment. If the interrupt happens pre increment it will have
> +	 * restored the counter when it returns.  We just need a barrier to
> +	 * keep gcc from moving things around.
>  	 */
>  	barrier();
> -	if (use_stack == 1) {
> -		trace.entries		= this_cpu_ptr(ftrace_stack.calls);
> -		trace.max_entries	= FTRACE_STACK_MAX_ENTRIES;
> -
> -		if (regs)
> -			save_stack_trace_regs(regs, &trace);
> -		else
> -			save_stack_trace(&trace);
> -
> -		if (trace.nr_entries > size)
> -			size = trace.nr_entries;
> -	} else
> -		/* From now on, use_stack is a boolean */
> -		use_stack = 0;
> +
> +	fstack = this_cpu_ptr(ftrace_stacks.stacks) + (stackidx - 1);

nit: it would be slightly less surprising if stackidx were 0-based:

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d3f6ec7eb729..4fc93004feab 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2798,10 +2798,10 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer,
 	 */
 	preempt_disable_notrace();
 
-	stackidx = __this_cpu_inc_return(ftrace_stack_reserve);
+	stackidx = __this_cpu_inc_return(ftrace_stack_reserve) - 1;
 
 	/* This should never happen. If it does, yell once and skip */
-	if (WARN_ON_ONCE(stackidx >= FTRACE_KSTACK_NESTING))
+	if (WARN_ON_ONCE(stackidx > FTRACE_KSTACK_NESTING))
 		goto out;
 
 	/*
@@ -2813,7 +2813,7 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer,
 	 */
 	barrier();
 
-	fstack = this_cpu_ptr(ftrace_stacks.stacks) + (stackidx - 1);
+	fstack = this_cpu_ptr(ftrace_stacks.stacks) + stackidx;
 	trace.entries		= fstack->calls;
 	trace.max_entries	= FTRACE_KSTACK_ENTRIES;