block: add one-hit cache for disk partition lookup

disk_map_sector_rcu() returns a partition from a sector offset,
which we use for IO statistics on a per-partition basis. The
lookup itself is an O(N) list lookup, where N is the number of
partitions. This actually hurts performance quite a bit, even
on the lower end partitions. On higher numbered partitions,
it can get pretty bad.

Solve this by adding a one-hit cache for partition lookup.
This makes the lookup O(1) for the case where we do most IO to
one partition. Even for mixed partition workloads, amortized cost
is pretty close to O(1) since the natural IO batching makes the
one-hit cache last for lots of IOs.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 19 insertions, 4 deletions

diff --git a/block/genhd.c b/block/genhd.c
index 2f7feda..d84a7df 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -181,6 +181,12 @@ void disk_part_iter_exit(struct disk_part_iter *piter)
 }
 EXPORT_SYMBOL_GPL(disk_part_iter_exit);
 
+static inline int sector_in_part(struct hd_struct *part, sector_t sector)
+{
+	return part->start_sect <= sector &&
+		sector < part->start_sect + part->nr_sects;
+}
+
 /**
  * disk_map_sector_rcu - map sector to partition
  * @disk: gendisk of interest
@@ -199,16 +205,22 @@ EXPORT_SYMBOL_GPL(disk_part_iter_exit);
 struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 {
 	struct disk_part_tbl *ptbl;
+	struct hd_struct *part;
 	int i;
 
 	ptbl = rcu_dereference(disk->part_tbl);
 
+	part = rcu_dereference(ptbl->last_lookup);
+	if (part && sector_in_part(part, sector))
+		return part;
+
 	for (i = 1; i < ptbl->len; i++) {
-		struct hd_struct *part = rcu_dereference(ptbl->part[i]);
+		part = rcu_dereference(ptbl->part[i]);
 
-		if (part && part->start_sect <= sector &&
-		    sector < part->start_sect + part->nr_sects)
+		if (part && sector_in_part(part, sector)) {
+			rcu_assign_pointer(ptbl->last_lookup, part);
 			return part;
+		}
 	}
 	return &disk->part0;
 }
@@ -888,8 +900,11 @@ static void disk_replace_part_tbl(struct gendisk *disk,
 	struct disk_part_tbl *old_ptbl = disk->part_tbl;
 
 	rcu_assign_pointer(disk->part_tbl, new_ptbl);
-	if (old_ptbl)
+
+	if (old_ptbl) {
+		rcu_assign_pointer(old_ptbl->last_lookup, NULL);
 		call_rcu(&old_ptbl->rcu_head, disk_free_ptbl_rcu_cb);
+	}
 }
 
 /**