xref: /freebsd-10.0-release/sys/geom/sched/subr_disk.c

/*-
 * ----------------------------------------------------------------------------
 * "THE BEER-WARE LICENSE" (Revision 42):
 * <phk@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
 * can do whatever you want with this stuff. If we meet some day, and you think
 * this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
 * ----------------------------------------------------------------------------
 *
 * The bioq_disksort() (and the specification of the bioq API)
 * have been written by Luigi Rizzo and Fabio Checconi under the same
 * license as above.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

//#include "opt_geom.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/bio.h>
#include <sys/conf.h>
#include <sys/disk.h>
#include <geom/geom_disk.h>
#include "g_sched.h"

/*
 * BIO queue implementation
 *
 * Please read carefully the description below before making any change
 * to the code, or you might change the behaviour of the data structure
 * in undesirable ways.
 *
 * A bioq stores disk I/O request (bio), normally sorted according to
 * the distance of the requested position (bio->bio_offset) from the
 * current head position (bioq->last_offset) in the scan direction, i.e.
 *
 * 	(uoff_t)(bio_offset - last_offset)
 *
 * Note that the cast to unsigned (uoff_t) is fundamental to insure
 * that the distance is computed in the scan direction.
 *
 * The main methods for manipulating the bioq are:
 *
 *   bioq_disksort()	performs an ordered insertion;
 *
 *   bioq_first()	return the head of the queue, without removing;
 *
 *   bioq_takefirst()	return and remove the head of the queue,
 *		updating the 'current head position' as
 *		bioq->last_offset = bio->bio_offset + bio->bio_length;
 *
 * When updating the 'current head position', we assume that the result of
 * bioq_takefirst() is dispatched to the device, so bioq->last_offset
 * represents the head position once the request is complete.
 *
 * If the bioq is manipulated using only the above calls, it starts
 * with a sorted sequence of requests with bio_offset >= last_offset,
 * possibly followed by another sorted sequence of requests with
 * 0 <= bio_offset < bioq->last_offset
 *
 * NOTE: historical behaviour was to ignore bio->bio_length in the
 *	update, but its use tracks the head position in a better way.
 *	Historical behaviour was also to update the head position when
 *	the request under service is complete, rather than when the
 *	request is extracted from the queue. However, the current API
 *	has no method to update the head position; secondly, once
 *	a request has been submitted to the disk, we have no idea of
 *	the actual head position, so the final one is our best guess.
 *
 * --- Direct queue manipulation ---
 *
 * A bioq uses an underlying TAILQ to store requests, so we also
 * export methods to manipulate the TAILQ, in particular:
 *
 * bioq_insert_tail()	insert an entry at the end.
 *		It also creates a 'barrier' so all subsequent
 *		insertions through bioq_disksort() will end up
 *		after this entry;
 *
 * bioq_insert_head()	insert an entry at the head, update
 *		bioq->last_offset = bio->bio_offset so that
 *		all subsequent insertions through bioq_disksort()
 *		will end up after this entry;
 *
 * bioq_remove()	remove a generic element from the queue, act as
 *		bioq_takefirst() if invoked on the head of the queue.
 *
 * The semantic of these methods is the same as the operations
 * on the underlying TAILQ, but with additional guarantees on
 * subsequent bioq_disksort() calls. E.g. bioq_insert_tail()
 * can be useful for making sure that all previous ops are flushed
 * to disk before continuing.
 *
 * Updating bioq->last_offset on a bioq_insert_head() guarantees
 * that the bio inserted with the last bioq_insert_head() will stay
 * at the head of the queue even after subsequent bioq_disksort().
 *
 * Note that when the direct queue manipulation functions are used,
 * the queue may contain multiple inversion points (i.e. more than
 * two sorted sequences of requests).
 *
 */

void
gs_bioq_init(struct bio_queue_head *head)
{

	TAILQ_INIT(&head->queue);
	head->last_offset = 0;
	head->insert_point = NULL;
}

void
gs_bioq_remove(struct bio_queue_head *head, struct bio *bp)
{

	if (head->insert_point == NULL) {
		if (bp == TAILQ_FIRST(&head->queue))
			head->last_offset = bp->bio_offset + bp->bio_length;
	} else if (bp == head->insert_point)
		head->insert_point = NULL;

	TAILQ_REMOVE(&head->queue, bp, bio_queue);
}

void
gs_bioq_flush(struct bio_queue_head *head, struct devstat *stp, int error)
{
	struct bio *bp;

	while ((bp = gs_bioq_takefirst(head)) != NULL)
		biofinish(bp, stp, error);
}

void
gs_bioq_insert_head(struct bio_queue_head *head, struct bio *bp)
{

	if (head->insert_point == NULL)
		head->last_offset = bp->bio_offset;
	TAILQ_INSERT_HEAD(&head->queue, bp, bio_queue);
}

void
gs_bioq_insert_tail(struct bio_queue_head *head, struct bio *bp)
{

	TAILQ_INSERT_TAIL(&head->queue, bp, bio_queue);
	head->insert_point = bp;
	head->last_offset = bp->bio_offset;
}

struct bio *
gs_bioq_first(struct bio_queue_head *head)
{

	return (TAILQ_FIRST(&head->queue));
}

struct bio *
gs_bioq_takefirst(struct bio_queue_head *head)
{
	struct bio *bp;

	bp = TAILQ_FIRST(&head->queue);
	if (bp != NULL)
		gs_bioq_remove(head, bp);
	return (bp);
}

/*
 * Compute the sorting key. The cast to unsigned is
 * fundamental for correctness, see the description
 * near the beginning of the file.
 */
static inline uoff_t
gs_bioq_bio_key(struct bio_queue_head *head, struct bio *bp)
{

	return ((uoff_t)(bp->bio_offset - head->last_offset));
}

/*
 * Seek sort for disks.
 *
 * Sort all requests in a single queue while keeping
 * track of the current position of the disk with last_offset.
 * See above for details.
 */
void
gs_bioq_disksort(struct bio_queue_head *head, struct bio *bp)
{
	struct bio *cur, *prev;
	uoff_t key;

	if ((bp->bio_flags & BIO_ORDERED) != 0) {
		/*
		 * Ordered transactions can only be dispatched
		 * after any currently queued transactions.  They
		 * also have barrier semantics - no transactions
		 * queued in the future can pass them.
		 */
		gs_bioq_insert_tail(head, bp);
		return;
	}

	prev = NULL;
	key = gs_bioq_bio_key(head, bp);
	cur = TAILQ_FIRST(&head->queue);

	if (head->insert_point) {
		prev = head->insert_point;
		cur = TAILQ_NEXT(head->insert_point, bio_queue);
	}

	while (cur != NULL && key >= gs_bioq_bio_key(head, cur)) {
		prev = cur;
		cur = TAILQ_NEXT(cur, bio_queue);
	}

	if (prev == NULL)
		TAILQ_INSERT_HEAD(&head->queue, bp, bio_queue);
	else
		TAILQ_INSERT_AFTER(&head->queue, prev, bp, bio_queue);
}