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ring-buffer: move big if statement down
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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: Steven Rostedt <rostedt@goodmis.org>
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Steven Rostedt authored and Steven Rostedt committed May 6, 2009
1 parent 94487d6 commit aa20ae8
Showing 1 changed file with 111 additions and 107 deletions.
218 changes: 111 additions & 107 deletions kernel/trace/ring_buffer.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -1173,137 +1174,140 @@ __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;

local_irq_save(flags);
/*
* Since the write to the buffer is still not
* fully lockless, we must be careful with NMIs.
* The locks in the writers are taken when a write
* crosses to a new page. The locks protect against
* races with the readers (this will soon be fixed
* with a lockless solution).
*
* Because we can not protect against NMIs, and we
* want to keep traces reentrant, we need to manage
* what happens when we are in an NMI.
*
* NMIs can happen after we take the lock.
* If we are in an NMI, only take the lock
* if it is not already taken. Otherwise
* simply fail.
*/
if (unlikely(in_nmi())) {
if (!__raw_spin_trylock(&cpu_buffer->lock)) {
cpu_buffer->nmi_dropped++;
goto out_reset;
}
} else
__raw_spin_lock(&cpu_buffer->lock);

lock_taken = true;
/* We reserved something on the buffer */

rb_inc_page(cpu_buffer, &next_page);
if (RB_WARN_ON(cpu_buffer, write > BUF_PAGE_SIZE))
return NULL;

head_page = cpu_buffer->head_page;
reader_page = cpu_buffer->reader_page;
event = __rb_page_index(tail_page, tail);
rb_update_event(event, type, length);

/* we grabbed the lock before incrementing */
if (RB_WARN_ON(cpu_buffer, next_page == reader_page))
goto out_reset;
/* The passed in type is zero for DATA */
if (likely(!type))
local_inc(&tail_page->entries);

/*
* If for some reason, we had an interrupt storm that made
* it all the way around the buffer, bail, and warn
* about it.
*/
if (unlikely(next_page == commit_page)) {
cpu_buffer->commit_overrun++;
goto out_reset;
}
/*
* 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;

if (next_page == head_page) {
if (!(buffer->flags & RB_FL_OVERWRITE))
goto out_reset;
return event;

/* tail_page has not moved yet? */
if (tail_page == cpu_buffer->tail_page) {
/* count overflows */
cpu_buffer->overrun +=
local_read(&head_page->entries);
next_page:

rb_inc_page(cpu_buffer, &head_page);
cpu_buffer->head_page = head_page;
cpu_buffer->head_page->read = 0;
}
}
next_page = tail_page;

/*
* If the tail page is still the same as what we think
* it is, then it is up to us to update the tail
* pointer.
*/
if (tail_page == cpu_buffer->tail_page) {
local_set(&next_page->write, 0);
local_set(&next_page->entries, 0);
local_set(&next_page->page->commit, 0);
cpu_buffer->tail_page = next_page;

/* reread the time stamp */
*ts = ring_buffer_time_stamp(buffer, cpu_buffer->cpu);
cpu_buffer->tail_page->page->time_stamp = *ts;
local_irq_save(flags);
/*
* Since the write to the buffer is still not
* fully lockless, we must be careful with NMIs.
* The locks in the writers are taken when a write
* crosses to a new page. The locks protect against
* races with the readers (this will soon be fixed
* with a lockless solution).
*
* Because we can not protect against NMIs, and we
* want to keep traces reentrant, we need to manage
* what happens when we are in an NMI.
*
* NMIs can happen after we take the lock.
* If we are in an NMI, only take the lock
* if it is not already taken. Otherwise
* simply fail.
*/
if (unlikely(in_nmi())) {
if (!__raw_spin_trylock(&cpu_buffer->lock)) {
cpu_buffer->nmi_dropped++;
goto out_reset;
}
} else
__raw_spin_lock(&cpu_buffer->lock);

/*
* The actual tail page has moved forward.
*/
if (tail < BUF_PAGE_SIZE) {
/* Mark the rest of the page with padding */
event = __rb_page_index(tail_page, tail);
rb_event_set_padding(event);
}
lock_taken = true;

if (tail <= BUF_PAGE_SIZE)
/* Set the write back to the previous setting */
local_set(&tail_page->write, tail);
rb_inc_page(cpu_buffer, &next_page);

/*
* If this was a commit entry that failed,
* increment that too
*/
if (tail_page == cpu_buffer->commit_page &&
tail == rb_commit_index(cpu_buffer)) {
rb_set_commit_to_write(cpu_buffer);
}
head_page = cpu_buffer->head_page;
reader_page = cpu_buffer->reader_page;

__raw_spin_unlock(&cpu_buffer->lock);
local_irq_restore(flags);
/* we grabbed the lock before incrementing */
if (RB_WARN_ON(cpu_buffer, next_page == reader_page))
goto out_reset;

/* fail and let the caller try again */
return ERR_PTR(-EAGAIN);
/*
* If for some reason, we had an interrupt storm that made
* it all the way around the buffer, bail, and warn
* about it.
*/
if (unlikely(next_page == commit_page)) {
cpu_buffer->commit_overrun++;
goto out_reset;
}

/* We reserved something on the buffer */
if (next_page == head_page) {
if (!(buffer->flags & RB_FL_OVERWRITE))
goto out_reset;

if (RB_WARN_ON(cpu_buffer, write > BUF_PAGE_SIZE))
return NULL;
/* tail_page has not moved yet? */
if (tail_page == cpu_buffer->tail_page) {
/* count overflows */
cpu_buffer->overrun +=
local_read(&head_page->entries);

event = __rb_page_index(tail_page, tail);
rb_update_event(event, type, length);
rb_inc_page(cpu_buffer, &head_page);
cpu_buffer->head_page = head_page;
cpu_buffer->head_page->read = 0;
}
}

/* The passed in type is zero for DATA */
if (likely(!type))
local_inc(&tail_page->entries);
/*
* If the tail page is still the same as what we think
* it is, then it is up to us to update the tail
* pointer.
*/
if (tail_page == cpu_buffer->tail_page) {
local_set(&next_page->write, 0);
local_set(&next_page->entries, 0);
local_set(&next_page->page->commit, 0);
cpu_buffer->tail_page = next_page;

/* reread the time stamp */
*ts = ring_buffer_time_stamp(buffer, cpu_buffer->cpu);
cpu_buffer->tail_page->page->time_stamp = *ts;
}

/*
* If this is a commit and the tail is zero, then update
* this page's time stamp.
* The actual tail page has moved forward.
*/
if (!tail && rb_is_commit(cpu_buffer, event))
cpu_buffer->commit_page->page->time_stamp = *ts;
if (tail < BUF_PAGE_SIZE) {
/* Mark the rest of the page with padding */
event = __rb_page_index(tail_page, tail);
rb_event_set_padding(event);
}

return event;
if (tail <= BUF_PAGE_SIZE)
/* Set the write back to the previous setting */
local_set(&tail_page->write, tail);

/*
* If this was a commit entry that failed,
* increment that too
*/
if (tail_page == cpu_buffer->commit_page &&
tail == rb_commit_index(cpu_buffer)) {
rb_set_commit_to_write(cpu_buffer);
}

__raw_spin_unlock(&cpu_buffer->lock);
local_irq_restore(flags);

/* fail and let the caller try again */
return ERR_PTR(-EAGAIN);

out_reset:
/* reset write */
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