Commit aa20ae84 authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
Browse files

ring-buffer: move big if statement down 



In the hot path of the ring buffer "__rb_reserve_next" there's a big
if statement that does not even return back to the work flow.

	code;

	if (cross to next page) {

		[ lots of code ]

		return;
	}

	more code;

The condition is even the unlikely path, although we do not denote it
with an unlikely because gcc is fine with it. The condition is true when
the write crosses a page boundary, and we need to start at a new page.

Having this if statement makes it hard to read, but calling another
function to do the work is also not appropriate, because we are using a lot
of variables that were set before the if statement, and we do not want to
send them as parameters.

This patch changes it to a goto:

	code;

	if (cross to next page)
		goto next_page;

	more code;

	return;

next_page:

	[ lots of code]

This makes the code easier to understand, and a bit more obvious.

The output from gcc is practically identical. For some reason, gcc decided
to use different registers when I switched it to a goto. But other than that,
the logic is the same.

[ Impact: easier to read code ]

Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent 94487d6d

kernel/trace/ring_buffer.c

+111 −107
Original line number Diff line number Diff line
@@ -1159,6 +1159,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, 
		  unsigned type, unsigned long length, u64 *ts)

{

	struct buffer_page *tail_page, *head_page, *reader_page, *commit_page;

	struct buffer_page *next_page;

	unsigned long tail, write;

	struct ring_buffer *buffer = cpu_buffer->buffer;

	struct ring_buffer_event *event;
@@ -1173,8 +1174,33 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, 
	tail = write - length;



	/* See if we shot pass the end of this buffer page */

	if (write > BUF_PAGE_SIZE) {

		struct buffer_page *next_page = tail_page;

	if (write > BUF_PAGE_SIZE)

		goto next_page;



	/* We reserved something on the buffer */



	if (RB_WARN_ON(cpu_buffer, write > BUF_PAGE_SIZE))

		return NULL;



	event = __rb_page_index(tail_page, tail);

	rb_update_event(event, type, length);



	/* The passed in type is zero for DATA */

	if (likely(!type))

		local_inc(&tail_page->entries);



	/*

	 * If this is a commit and the tail is zero, then update

	 * this page's time stamp.

	 */

	if (!tail && rb_is_commit(cpu_buffer, event))

		cpu_buffer->commit_page->page->time_stamp = *ts;



	return event;



 next_page:



	next_page = tail_page;



	local_irq_save(flags);

	/*
@@ -1282,28 +1308,6 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, 


	/* fail and let the caller try again */

	return ERR_PTR(-EAGAIN);

	}



	/* We reserved something on the buffer */



	if (RB_WARN_ON(cpu_buffer, write > BUF_PAGE_SIZE))

		return NULL;



	event = __rb_page_index(tail_page, tail);

	rb_update_event(event, type, length);



	/* The passed in type is zero for DATA */

	if (likely(!type))

		local_inc(&tail_page->entries);



	/*

	 * If this is a commit and the tail is zero, then update

	 * this page's time stamp.

	 */

	if (!tail && rb_is_commit(cpu_buffer, event))

		cpu_buffer->commit_page->page->time_stamp = *ts;



	return event;



 out_reset:

	/* reset write */

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