ring-buffer: only enable ring_buffer_swap_cpu when needed

Since the ability to swap the cpu buffers adds a small overhead to
the recording of a trace, we only want to add it when needed.

Only the irqsoff and preemptoff tracers use this feature, and both are
not recommended for production kernels. This patch disables its use
when neither irqsoff nor preemptoff is configured.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/linux/ring_buffer.h

@@ -140,8 +140,17 @@ unsigned long ring_buffer_size(struct ring_buffer *buffer);
 void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu);
 void ring_buffer_reset(struct ring_buffer *buffer);
 
+#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
 int ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
 			 struct ring_buffer *buffer_b, int cpu);
+#else
+static inline int
+ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
+		     struct ring_buffer *buffer_b, int cpu)
+{
+	return -ENODEV;
+}
+#endif
 
 int ring_buffer_empty(struct ring_buffer *buffer);
 int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu);

kernel/trace/Kconfig

@@ -62,6 +62,12 @@ config EVENT_TRACING
 config CONTEXT_SWITCH_TRACER
 	bool
 
+config RING_BUFFER_ALLOW_SWAP
+	bool
+	help
+	 Allow the use of ring_buffer_swap_cpu.
+	 Adds a very slight overhead to tracing when enabled.
+
 # All tracer options should select GENERIC_TRACER. For those options that are
 # enabled by all tracers (context switch and event tracer) they select TRACING.
 # This allows those options to appear when no other tracer is selected. But the
@@ -146,6 +152,7 @@ config IRQSOFF_TRACER
 	select TRACE_IRQFLAGS
 	select GENERIC_TRACER
 	select TRACER_MAX_TRACE
+	select RING_BUFFER_ALLOW_SWAP
 	help
 	  This option measures the time spent in irqs-off critical
 	  sections, with microsecond accuracy.
@@ -167,6 +174,7 @@ config PREEMPT_TRACER
 	depends on PREEMPT
 	select GENERIC_TRACER
 	select TRACER_MAX_TRACE
+	select RING_BUFFER_ALLOW_SWAP
 	help
 	  This option measures the time spent in preemption off critical
 	  sections, with microsecond accuracy.

kernel/trace/ring_buffer.c

@@ -2084,6 +2084,7 @@ rb_reserve_next_event(struct ring_buffer *buffer,
 
 	rb_start_commit(cpu_buffer);
 
+#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
 	/*
 	 * Due to the ability to swap a cpu buffer from a buffer
 	 * it is possible it was swapped before we committed.
@@ -2096,6 +2097,7 @@ rb_reserve_next_event(struct ring_buffer *buffer,
 		local_dec(&cpu_buffer->commits);
 		return NULL;
 	}
+#endif
 
 	length = rb_calculate_event_length(length);
  again:
@@ -3498,6 +3500,7 @@ int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_empty_cpu);
 
+#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
 /**
  * ring_buffer_swap_cpu - swap a CPU buffer between two ring buffers
  * @buffer_a: One buffer to swap with
@@ -3573,6 +3576,7 @@ int ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
 	return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
+#endif /* CONFIG_RING_BUFFER_ALLOW_SWAP */
 
 /**
  * ring_buffer_alloc_read_page - allocate a page to read from buffer