dm thin: sort the per thin deferred bios using an rb_tree

A thin-pool will allocate blocks using FIFO order for all thin devices
which share the thin-pool.  Because of this simplistic allocation the
thin-pool's space can become fragmented quite easily; especially when
multiple threads are requesting blocks in parallel.

Sort each thin device's deferred_bio_list based on logical sector to
help reduce fragmentation of the thin-pool's ondisk layout.

The following tables illustrate the realized gains/potential offered by
sorting each thin device's deferred_bio_list.  An "io size"-sized random
read of the device would result in "seeks/io" fragments being read, with
an average "distance/seek" between each fragment.

Data was written to a single thin device using multiple threads via
iozone (8 threads, 64K for both the block_size and io_size).

unsorted:

     io size   seeks/io distance/seek
  --------------------------------------
          4k    0.000   0b
         16k    0.013   11m
         64k    0.065   11m
        256k    0.274   10m
          1m    1.109   10m
          4m    4.411   10m
         16m    17.097  11m
         64m    60.055  13m
        256m    148.798 25m
          1g    809.929 21m

sorted:

     io size   seeks/io distance/seek
  --------------------------------------
          4k    0.000   0b
         16k    0.000   1g
         64k    0.001   1g
        256k    0.003   1g
          1m    0.011   1g
          4m    0.045   1g
         16m    0.181   1g
         64m    0.747   1011m
        256m    3.299   1g
          1g    14.373  1g

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

1 files changed, 82 insertions, 2 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 08e62aef361d..53728be84dee 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -16,6 +16,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/slab.h>
+#include <linux/rbtree.h>
 
 #define DM_MSG_PREFIX   "thin"
 
@@ -230,6 +231,7 @@ struct thin_c {
         spinlock_t lock;
         struct bio_list deferred_bio_list;
         struct bio_list retry_on_resume_list;
+        struct rb_root sort_bio_list; /* sorted list of deferred bios */
 };
 
 /*----------------------------------------------------------------*/
@@ -371,6 +373,7 @@ struct dm_thin_endio_hook {
         struct dm_deferred_entry *shared_read_entry;
         struct dm_deferred_entry *all_io_entry;
         struct dm_thin_new_mapping *overwrite_mapping;
+        struct rb_node rb_node;
 };
 
 static void requeue_bio_list(struct thin_c *tc, struct bio_list *master)
@@ -1367,12 +1370,77 @@ static int need_commit_due_to_time(struct pool *pool)
                jiffies > pool->last_commit_jiffies + COMMIT_PERIOD;
 }
 
+#define thin_pbd(node) rb_entry((node), struct dm_thin_endio_hook, rb_node)
+#define thin_bio(pbd) dm_bio_from_per_bio_data((pbd), sizeof(struct dm_thin_endio_hook))
+
+static void __thin_bio_rb_add(struct thin_c *tc, struct bio *bio)
+{
+        struct rb_node **rbp, *parent;
+        struct dm_thin_endio_hook *pbd;
+        sector_t bi_sector = bio->bi_iter.bi_sector;
+
+        rbp = &tc->sort_bio_list.rb_node;
+        parent = NULL;
+        while (*rbp) {
+                parent = *rbp;
+                pbd = thin_pbd(parent);
+
+                if (bi_sector < thin_bio(pbd)->bi_iter.bi_sector)
+                        rbp = &(*rbp)->rb_left;
+                else
+                        rbp = &(*rbp)->rb_right;
+        }
+
+        pbd = dm_per_bio_data(bio, sizeof(struct dm_thin_endio_hook));
+        rb_link_node(&pbd->rb_node, parent, rbp);
+        rb_insert_color(&pbd->rb_node, &tc->sort_bio_list);
+}
+
+static void __extract_sorted_bios(struct thin_c *tc)
+{
+        struct rb_node *node;
+        struct dm_thin_endio_hook *pbd;
+        struct bio *bio;
+
+        for (node = rb_first(&tc->sort_bio_list); node; node = rb_next(node)) {
+                pbd = thin_pbd(node);
+                bio = thin_bio(pbd);
+
+                bio_list_add(&tc->deferred_bio_list, bio);
+                rb_erase(&pbd->rb_node, &tc->sort_bio_list);
+        }
+
+        WARN_ON(!RB_EMPTY_ROOT(&tc->sort_bio_list));
+}
+
+static void __sort_thin_deferred_bios(struct thin_c *tc)
+{
+        struct bio *bio;
+        struct bio_list bios;
+
+        bio_list_init(&bios);
+        bio_list_merge(&bios, &tc->deferred_bio_list);
+        bio_list_init(&tc->deferred_bio_list);
+
+        /* Sort deferred_bio_list using rb-tree */
+        while ((bio = bio_list_pop(&bios)))
+                __thin_bio_rb_add(tc, bio);
+
+        /*
+         * Transfer the sorted bios in sort_bio_list back to
+         * deferred_bio_list to allow lockless submission of
+         * all bios.
+         */
+        __extract_sorted_bios(tc);
+}
+
 static void process_thin_deferred_bios(struct thin_c *tc)
 {
         struct pool *pool = tc->pool;
         unsigned long flags;
         struct bio *bio;
         struct bio_list bios;
+        struct blk_plug plug;
 
         if (tc->requeue_mode) {
                 requeue_bio_list(tc, &tc->deferred_bio_list);
@@ -1382,10 +1450,20 @@ static void process_thin_deferred_bios(struct thin_c *tc)
         bio_list_init(&bios);
 
         spin_lock_irqsave(&tc->lock, flags);
+
+        if (bio_list_empty(&tc->deferred_bio_list)) {
+                spin_unlock_irqrestore(&tc->lock, flags);
+                return;
+        }
+
+        __sort_thin_deferred_bios(tc);
+
         bio_list_merge(&bios, &tc->deferred_bio_list);
         bio_list_init(&tc->deferred_bio_list);
+
         spin_unlock_irqrestore(&tc->lock, flags);
 
+        blk_start_plug(&plug);
         while ((bio = bio_list_pop(&bios))) {
                 /*
                  * If we've got no free new_mapping structs, and processing
@@ -1405,6 +1483,7 @@ static void process_thin_deferred_bios(struct thin_c *tc)
                 else
                         pool->process_bio(tc, bio);
         }
+        blk_finish_plug(&plug);
 }
 
 static void process_deferred_bios(struct pool *pool)
@@ -2964,7 +3043,7 @@ static struct target_type pool_target = {
         .name = "thin-pool",
         .features = DM_TARGET_SINGLETON | DM_TARGET_ALWAYS_WRITEABLE |
                     DM_TARGET_IMMUTABLE,
-        .version = {1, 11, 0},
+        .version = {1, 12, 0},
         .module = THIS_MODULE,
         .ctr = pool_ctr,
         .dtr = pool_dtr,
@@ -3040,6 +3119,7 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
         spin_lock_init(&tc->lock);
         bio_list_init(&tc->deferred_bio_list);
         bio_list_init(&tc->retry_on_resume_list);
+        tc->sort_bio_list = RB_ROOT;
 
         if (argc == 3) {
                 r = dm_get_device(ti, argv[2], FMODE_READ, &origin_dev);
@@ -3287,7 +3367,7 @@ static int thin_iterate_devices(struct dm_target *ti,
 
 static struct target_type thin_target = {
         .name = "thin",
-        .version = {1, 11, 0},
+        .version = {1, 12, 0},
         .module = THIS_MODULE,
         .ctr = thin_ctr,
         .dtr = thin_dtr,