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

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
......@@ -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];
};
......@@ -137,12 +142,21 @@ 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)
{
return q->nr_elts == 0;
......@@ -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;
......
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