ftrace: show buffer size in kilobytes

Impact: change the units of buffer_size_kb to kilobytes

This patch changes the units of the buffer_size_kb file to kilobytes.
Reading and writing to the file uses kilobytes as units. To help
users to know what units are used, the output of the file now
looks like:

  # cat /debug/tracing/buffer_size_kb
  1408

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/ftrace.txt

@@ -94,10 +94,10 @@ of ftrace. Here is a list of some of the key files:
 		only be recorded if the latency is greater than
 		the value in this file. (in microseconds)
 
-  buffer_size_kb: This sets or displays the number of bytes each CPU
+  buffer_size_kb: This sets or displays the number of kilobytes each CPU
 		buffer can hold. The tracer buffers are the same size
 		for each CPU. The displayed number is the size of the
-		 CPU buffer and not total size of all buffers. The
+		CPU buffer and not total size of all buffers. The
 		trace buffers are allocated in pages (blocks of memory
 		that the kernel uses for allocation, usually 4 KB in size).
 		If the last page allocated has room for more bytes
@@ -1306,28 +1306,16 @@ the full size, multiply the number of possible CPUS with the
 number of entries.
 
  # cat /debug/tracing/buffer_size_kb
-65620
+1408 (units kilobytes)
 
 Note, to modify this, you must have tracing completely disabled. To do that,
 echo "nop" into the current_tracer. If the current_tracer is not set
 to "nop", an EINVAL error will be returned.
 
  # echo nop > /debug/tracing/current_tracer
- # echo 100000 > /debug/tracing/buffer_size_kb
+ # echo 10000 > /debug/tracing/buffer_size_kb
  # cat /debug/tracing/buffer_size_kb
-100045
-
-
-Notice that we echoed in 100,000 but the size is 100,045. The entries
-are held in individual pages. It allocates the number of pages it takes
-to fulfill the request. If more entries may fit on the last page
-then they will be added.
-
- # echo 1 > /debug/tracing/buffer_size_kb
- # cat /debug/tracing/buffer_size_kb
-85
-
-This shows us that 85 entries can fit in a single page.
+10000 (units kilobytes)
 
 The number of pages which will be allocated is limited to a percentage
 of available memory. Allocating too much will produce an error.

kernel/trace/trace.c

@@ -2905,7 +2905,7 @@ tracing_entries_read(struct file *filp, char __user *ubuf,
 	char buf[64];
 	int r;
 
-	r = sprintf(buf, "%lu\n", tr->entries);
+	r = sprintf(buf, "%lu\n", tr->entries >> 10);
 	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
 }
 
@@ -2945,6 +2945,9 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 			atomic_inc(&max_tr.data[cpu]->disabled);
 	}
 
+	/* value is in KB */
+	val <<= 10;
+
 	if (val != global_trace.entries) {
 		ret = ring_buffer_resize(global_trace.buffer, val);
 		if (ret < 0) {