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Commit e65ff870 authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
Browse files

dm cache policy mq: try not to writeback data that changed in the last second 



Writeback takes out a lock on the cache block, so will increase the
latency for any concurrent io.

This patch works by placing 2 sentinel objects on each level of the
multiqueues.  Every WRITEBACK_PERIOD the oldest sentinel gets moved to
the newest end of the queue level.

When looking for writeback work:
  if less than 25% of the cache is clean:
    we select the oldest object with the lowest hit count
  otherwise:
    we select the oldest object that is not past a writeback sentinel.

Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent fdecee32

drivers/md/dm-cache-policy-mq.c

+93 −1
Original line number Diff line number Diff line
@@ -8,6 +8,7 @@ 
#include "dm.h"



#include <linux/hash.h>

#include <linux/jiffies.h>

#include <linux/module.h>

#include <linux/mutex.h>

#include <linux/slab.h>
@@ -126,8 +127,12 @@ static void iot_examine_bio(struct io_tracker *t, struct bio *bio) 
#define NR_QUEUE_LEVELS 16u

#define NR_SENTINELS NR_QUEUE_LEVELS * 3



#define WRITEBACK_PERIOD HZ



struct queue {

	unsigned nr_elts;

	bool current_writeback_sentinels;

	unsigned long next_writeback;

	struct list_head qs[NR_QUEUE_LEVELS];

	struct list_head sentinels[NR_SENTINELS];

};
@@ -137,10 +142,19 @@ static void queue_init(struct queue *q) 
	unsigned i;



	q->nr_elts = 0;

	q->current_writeback_sentinels = false;

	q->next_writeback = 0;

	for (i = 0; i < NR_QUEUE_LEVELS; i++) {

		INIT_LIST_HEAD(q->qs + i);

		INIT_LIST_HEAD(q->sentinels + i);

		INIT_LIST_HEAD(q->sentinels + NR_QUEUE_LEVELS + i);

		INIT_LIST_HEAD(q->sentinels + (2 * NR_QUEUE_LEVELS) + i);

	}

}



static unsigned queue_size(struct queue *q)

{

	return q->nr_elts;

}



static bool queue_empty(struct queue *q)
@@ -197,6 +211,27 @@ static struct list_head *queue_pop(struct queue *q) 
	return r;

}



/*

 * Pops an entry from a level that is not past a sentinel.

 */

static struct list_head *queue_pop_old(struct queue *q)

{

	unsigned level;

	struct list_head *h;



	for (level = 0; level < NR_QUEUE_LEVELS; level++)

		list_for_each(h, q->qs + level) {

			if (is_sentinel(q, h))

				break;



			q->nr_elts--;

			list_del(h);

			return h;

		}



	return NULL;

}



static struct list_head *list_pop(struct list_head *lh)

{

	struct list_head *r = lh->next;
@@ -207,6 +242,31 @@ static struct list_head *list_pop(struct list_head *lh) 
	return r;

}



static struct list_head *writeback_sentinel(struct queue *q, unsigned level)

{

	if (q->current_writeback_sentinels)

		return q->sentinels + NR_QUEUE_LEVELS + level;

	else

		return q->sentinels + 2 * NR_QUEUE_LEVELS + level;

}



static void queue_update_writeback_sentinels(struct queue *q)

{

	unsigned i;

	struct list_head *h;



	if (time_after(jiffies, q->next_writeback)) {

		for (i = 0; i < NR_QUEUE_LEVELS; i++) {

			h = writeback_sentinel(q, i);

			list_del(h);

			list_add_tail(h, q->qs + i);

		}



		q->next_writeback = jiffies + WRITEBACK_PERIOD;

		q->current_writeback_sentinels = !q->current_writeback_sentinels;

	}

}



/*

 * Sometimes we want to iterate through entries that have been pushed since

 * a certain event.  We use sentinel entries on the queues to delimit these
@@ -540,6 +600,20 @@ static struct entry *pop(struct mq_policy *mq, struct queue *q) 
	return e;

}



static struct entry *pop_old(struct mq_policy *mq, struct queue *q)

{

	struct entry *e;

	struct list_head *h = queue_pop_old(q);



	if (!h)

		return NULL;



	e = container_of(h, struct entry, list);

	hash_remove(e);



	return e;

}



static struct entry *peek(struct queue *q)

{

	struct list_head *h = queue_peek(q);
@@ -932,6 +1006,7 @@ static void copy_tick(struct mq_policy *mq) 
	queue_tick(&mq->pre_cache);

	queue_tick(&mq->cache_dirty);

	queue_tick(&mq->cache_clean);

	queue_update_writeback_sentinels(&mq->cache_dirty);

	spin_unlock_irqrestore(&mq->tick_lock, flags);

}
@@ -1112,10 +1187,27 @@ static int mq_remove_cblock(struct dm_cache_policy *p, dm_cblock_t cblock) 
	return r;

}



#define CLEAN_TARGET_PERCENTAGE 25



static bool clean_target_met(struct mq_policy *mq)

{

	/*

	 * Cache entries may not be populated.  So we're cannot rely on the

	 * size of the clean queue.

	 */

	unsigned nr_clean = from_cblock(mq->cache_size) - queue_size(&mq->cache_dirty);

	unsigned target = from_cblock(mq->cache_size) * CLEAN_TARGET_PERCENTAGE / 100;



	return nr_clean >= target;

}



static int __mq_writeback_work(struct mq_policy *mq, dm_oblock_t *oblock,

			      dm_cblock_t *cblock)

{

	struct entry *e = pop(mq, &mq->cache_dirty);

	struct entry *e = pop_old(mq, &mq->cache_dirty);



	if (!e && !clean_target_met(mq))

		e = pop(mq, &mq->cache_dirty);



	if (!e)

		return -ENODATA;