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>
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 @@
}
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 @@
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);
- for (i = 1; i < ptbl->len; i++) {
- struct hd_struct *part = rcu_dereference(ptbl->part[i]);
+ part = rcu_dereference(ptbl->last_lookup);
+ if (part && sector_in_part(part, sector))
+ return part;
- if (part && part->start_sect <= sector &&
- sector < part->start_sect + part->nr_sects)
+ for (i = 1; i < ptbl->len; i++) {
+ part = rcu_dereference(ptbl->part[i]);
+
+ if (part && sector_in_part(part, sector)) {
+ rcu_assign_pointer(ptbl->last_lookup, part);
return part;
+ }
}
return &disk->part0;
}
@@ -888,8 +900,11 @@
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);
+ }
}
/**
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 3df7742..16948ea 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -126,6 +126,7 @@
struct disk_part_tbl {
struct rcu_head rcu_head;
int len;
+ struct hd_struct *last_lookup;
struct hd_struct *part[];
};
