[PATCH] optimise loop driver a bit
Looks like locking can be optimised quite a lot. Increase lock widths
slightly so lo_lock is taken fewer times per request. Also it was quite
trivial to cover lo_pending with that lock, and remove the atomic
requirement. This also makes memory ordering explicitly correct, which is
nice (not that I particularly saw any mem ordering bugs).
Test was reading 4 250MB files in parallel on ext2-on-tmpfs filesystem (1K
block size, 4K page size). System is 2 socket Xeon with HT (4 thread).
intel:/home/npiggin# umount /dev/loop0 ; mount /dev/loop0 /mnt/loop ; /usr/bin/time ./mtloop.sh
Before:
0.24user 5.51system 0:02.84elapsed 202%CPU (0avgtext+0avgdata 0maxresident)k
0.19user 5.52system 0:02.88elapsed 198%CPU (0avgtext+0avgdata 0maxresident)k
0.19user 5.57system 0:02.89elapsed 198%CPU (0avgtext+0avgdata 0maxresident)k
0.22user 5.51system 0:02.90elapsed 197%CPU (0avgtext+0avgdata 0maxresident)k
0.19user 5.44system 0:02.91elapsed 193%CPU (0avgtext+0avgdata 0maxresident)k
After:
0.07user 2.34system 0:01.68elapsed 143%CPU (0avgtext+0avgdata 0maxresident)k
0.06user 2.37system 0:01.68elapsed 144%CPU (0avgtext+0avgdata 0maxresident)k
0.06user 2.39system 0:01.68elapsed 145%CPU (0avgtext+0avgdata 0maxresident)k
0.06user 2.36system 0:01.68elapsed 144%CPU (0avgtext+0avgdata 0maxresident)k
0.06user 2.42system 0:01.68elapsed 147%CPU (0avgtext+0avgdata 0maxresident)k
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 6f011d0..b35e088 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -472,17 +472,11 @@
*/
static void loop_add_bio(struct loop_device *lo, struct bio *bio)
{
- unsigned long flags;
-
- spin_lock_irqsave(&lo->lo_lock, flags);
if (lo->lo_biotail) {
lo->lo_biotail->bi_next = bio;
lo->lo_biotail = bio;
} else
lo->lo_bio = lo->lo_biotail = bio;
- spin_unlock_irqrestore(&lo->lo_lock, flags);
-
- up(&lo->lo_bh_mutex);
}
/*
@@ -492,14 +486,12 @@
{
struct bio *bio;
- spin_lock_irq(&lo->lo_lock);
if ((bio = lo->lo_bio)) {
if (bio == lo->lo_biotail)
lo->lo_biotail = NULL;
lo->lo_bio = bio->bi_next;
bio->bi_next = NULL;
}
- spin_unlock_irq(&lo->lo_lock);
return bio;
}
@@ -509,35 +501,28 @@
struct loop_device *lo = q->queuedata;
int rw = bio_rw(old_bio);
- if (!lo)
- goto out;
+ if (rw == READA)
+ rw = READ;
+
+ BUG_ON(!lo || (rw != READ && rw != WRITE));
spin_lock_irq(&lo->lo_lock);
if (lo->lo_state != Lo_bound)
- goto inactive;
- atomic_inc(&lo->lo_pending);
- spin_unlock_irq(&lo->lo_lock);
-
- if (rw == WRITE) {
- if (lo->lo_flags & LO_FLAGS_READ_ONLY)
- goto err;
- } else if (rw == READA) {
- rw = READ;
- } else if (rw != READ) {
- printk(KERN_ERR "loop: unknown command (%x)\n", rw);
- goto err;
- }
+ goto out;
+ if (unlikely(rw == WRITE && (lo->lo_flags & LO_FLAGS_READ_ONLY)))
+ goto out;
+ lo->lo_pending++;
loop_add_bio(lo, old_bio);
+ spin_unlock_irq(&lo->lo_lock);
+ up(&lo->lo_bh_mutex);
return 0;
-err:
- if (atomic_dec_and_test(&lo->lo_pending))
- up(&lo->lo_bh_mutex);
+
out:
+ if (lo->lo_pending == 0)
+ up(&lo->lo_bh_mutex);
+ spin_unlock_irq(&lo->lo_lock);
bio_io_error(old_bio, old_bio->bi_size);
return 0;
-inactive:
- spin_unlock_irq(&lo->lo_lock);
- goto out;
}
/*
@@ -560,13 +545,11 @@
static inline void loop_handle_bio(struct loop_device *lo, struct bio *bio)
{
- int ret;
-
if (unlikely(!bio->bi_bdev)) {
do_loop_switch(lo, bio->bi_private);
bio_put(bio);
} else {
- ret = do_bio_filebacked(lo, bio);
+ int ret = do_bio_filebacked(lo, bio);
bio_endio(bio, bio->bi_size, ret);
}
}
@@ -594,7 +577,7 @@
set_user_nice(current, -20);
lo->lo_state = Lo_bound;
- atomic_inc(&lo->lo_pending);
+ lo->lo_pending = 1;
/*
* up sem, we are running
@@ -602,26 +585,37 @@
up(&lo->lo_sem);
for (;;) {
- down_interruptible(&lo->lo_bh_mutex);
+ int pending;
+
/*
- * could be upped because of tear-down, not because of
- * pending work
+ * interruptible just to not contribute to load avg
*/
- if (!atomic_read(&lo->lo_pending))
+ if (down_interruptible(&lo->lo_bh_mutex))
+ continue;
+
+ spin_lock_irq(&lo->lo_lock);
+
+ /*
+ * could be upped because of tear-down, not pending work
+ */
+ if (unlikely(!lo->lo_pending)) {
+ spin_unlock_irq(&lo->lo_lock);
break;
+ }
bio = loop_get_bio(lo);
- if (!bio) {
- printk("loop: missing bio\n");
- continue;
- }
+ lo->lo_pending--;
+ pending = lo->lo_pending;
+ spin_unlock_irq(&lo->lo_lock);
+
+ BUG_ON(!bio);
loop_handle_bio(lo, bio);
/*
* upped both for pending work and tear-down, lo_pending
* will hit zero then
*/
- if (atomic_dec_and_test(&lo->lo_pending))
+ if (unlikely(!pending))
break;
}
@@ -900,7 +894,8 @@
spin_lock_irq(&lo->lo_lock);
lo->lo_state = Lo_rundown;
- if (atomic_dec_and_test(&lo->lo_pending))
+ lo->lo_pending--;
+ if (!lo->lo_pending)
up(&lo->lo_bh_mutex);
spin_unlock_irq(&lo->lo_lock);
