dm bufio: submit writes outside lock

This patch changes dm-bufio so that it submits write I/Os outside of the
lock. If the number of submitted buffers is greater than the number of
requests on the target queue, submit_bio blocks. We want to block outside
of the lock to improve latency of other threads that may need the lock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 58 insertions, 15 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 9588b864d311..5227e079a6e3 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -145,6 +145,7 @@ struct dm_buffer {
         unsigned long state;
         unsigned long last_accessed;
         struct dm_bufio_client *c;
+        struct list_head write_list;
         struct bio bio;
         struct bio_vec bio_vec[DM_BUFIO_INLINE_VECS];
 };
@@ -630,7 +631,8 @@ static int do_io_schedule(void *word)
  * - Submit our write and don't wait on it. We set B_WRITING indicating
  *   that there is a write in progress.
  */
-static void __write_dirty_buffer(struct dm_buffer *b)
+static void __write_dirty_buffer(struct dm_buffer *b,
+                                 struct list_head *write_list)
 {
         if (!test_bit(B_DIRTY, &b->state))
                 return;
@@ -639,7 +641,24 @@ static void __write_dirty_buffer(struct dm_buffer *b)
         wait_on_bit_lock(&b->state, B_WRITING,
                          do_io_schedule, TASK_UNINTERRUPTIBLE);
 
-        submit_io(b, WRITE, b->block, write_endio);
+        if (!write_list)
+                submit_io(b, WRITE, b->block, write_endio);
+        else
+                list_add_tail(&b->write_list, write_list);
+}
+
+static void __flush_write_list(struct list_head *write_list)
+{
+        struct blk_plug plug;
+        blk_start_plug(&plug);
+        while (!list_empty(write_list)) {
+                struct dm_buffer *b =
+                        list_entry(write_list->next, struct dm_buffer, write_list);
+                list_del(&b->write_list);
+                submit_io(b, WRITE, b->block, write_endio);
+                dm_bufio_cond_resched();
+        }
+        blk_finish_plug(&plug);
 }
 
 /*
@@ -655,7 +674,7 @@ static void __make_buffer_clean(struct dm_buffer *b)
                 return;
 
         wait_on_bit(&b->state, B_READING, do_io_schedule, TASK_UNINTERRUPTIBLE);
-        __write_dirty_buffer(b);
+        __write_dirty_buffer(b, NULL);
         wait_on_bit(&b->state, B_WRITING, do_io_schedule, TASK_UNINTERRUPTIBLE);
 }
 
@@ -802,7 +821,8 @@ static void __free_buffer_wake(struct dm_buffer *b)
         wake_up(&c->free_buffer_wait);
 }
 
-static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait)
+static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait,
+                                        struct list_head *write_list)
 {
         struct dm_buffer *b, *tmp;
 
@@ -818,7 +838,7 @@ static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait)
                 if (no_wait && test_bit(B_WRITING, &b->state))
                         return;
 
-                __write_dirty_buffer(b);
+                __write_dirty_buffer(b, write_list);
                 dm_bufio_cond_resched();
         }
 }
@@ -853,7 +873,8 @@ static void __get_memory_limit(struct dm_bufio_client *c,
  * If we are over threshold_buffers, start freeing buffers.
  * If we're over "limit_buffers", block until we get under the limit.
  */
-static void __check_watermark(struct dm_bufio_client *c)
+static void __check_watermark(struct dm_bufio_client *c,
+                              struct list_head *write_list)
 {
         unsigned long threshold_buffers, limit_buffers;
 
@@ -872,7 +893,7 @@ static void __check_watermark(struct dm_bufio_client *c)
         }
 
         if (c->n_buffers[LIST_DIRTY] > threshold_buffers)
-                __write_dirty_buffers_async(c, 1);
+                __write_dirty_buffers_async(c, 1, write_list);
 }
 
 /*
@@ -897,7 +918,8 @@ static struct dm_buffer *__find(struct dm_bufio_client *c, sector_t block)
  *--------------------------------------------------------------*/
 
 static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
-                                     enum new_flag nf, int *need_submit)
+                                     enum new_flag nf, int *need_submit,
+                                     struct list_head *write_list)
 {
         struct dm_buffer *b, *new_b = NULL;
 
@@ -924,7 +946,7 @@ static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
                 goto found_buffer;
         }
 
-        __check_watermark(c);
+        __check_watermark(c, write_list);
 
         b = new_b;
         b->hold_count = 1;
@@ -992,10 +1014,14 @@ static void *new_read(struct dm_bufio_client *c, sector_t block,
         int need_submit;
         struct dm_buffer *b;
 
+        LIST_HEAD(write_list);
+
         dm_bufio_lock(c);
-        b = __bufio_new(c, block, nf, &need_submit);
+        b = __bufio_new(c, block, nf, &need_submit, &write_list);
         dm_bufio_unlock(c);
 
+        __flush_write_list(&write_list);
+
         if (!b)
                 return b;
 
@@ -1047,6 +1073,8 @@ void dm_bufio_prefetch(struct dm_bufio_client *c,
 {
         struct blk_plug plug;
 
+        LIST_HEAD(write_list);
+
         BUG_ON(dm_bufio_in_request());
 
         blk_start_plug(&plug);
@@ -1055,7 +1083,15 @@ void dm_bufio_prefetch(struct dm_bufio_client *c,
         for (; n_blocks--; block++) {
                 int need_submit;
                 struct dm_buffer *b;
-                b = __bufio_new(c, block, NF_PREFETCH, &need_submit);
+                b = __bufio_new(c, block, NF_PREFETCH, &need_submit,
+                                &write_list);
+                if (unlikely(!list_empty(&write_list))) {
+                        dm_bufio_unlock(c);
+                        blk_finish_plug(&plug);
+                        __flush_write_list(&write_list);
+                        blk_start_plug(&plug);
+                        dm_bufio_lock(c);
+                }
                 if (unlikely(b != NULL)) {
                         dm_bufio_unlock(c);
 
@@ -1069,7 +1105,6 @@ void dm_bufio_prefetch(struct dm_bufio_client *c,
                                 goto flush_plug;
                         dm_bufio_lock(c);
                 }
-
         }
 
         dm_bufio_unlock(c);
@@ -1126,11 +1161,14 @@ EXPORT_SYMBOL_GPL(dm_bufio_mark_buffer_dirty);
 
 void dm_bufio_write_dirty_buffers_async(struct dm_bufio_client *c)
 {
+        LIST_HEAD(write_list);
+
         BUG_ON(dm_bufio_in_request());
 
         dm_bufio_lock(c);
-        __write_dirty_buffers_async(c, 0);
+        __write_dirty_buffers_async(c, 0, &write_list);
         dm_bufio_unlock(c);
+        __flush_write_list(&write_list);
 }
 EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers_async);
 
@@ -1147,8 +1185,13 @@ int dm_bufio_write_dirty_buffers(struct dm_bufio_client *c)
         unsigned long buffers_processed = 0;
         struct dm_buffer *b, *tmp;
 
+        LIST_HEAD(write_list);
+
+        dm_bufio_lock(c);
+        __write_dirty_buffers_async(c, 0, &write_list);
+        dm_bufio_unlock(c);
+        __flush_write_list(&write_list);
         dm_bufio_lock(c);
-        __write_dirty_buffers_async(c, 0);
 
 again:
         list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_DIRTY], lru_list) {
@@ -1274,7 +1317,7 @@ retry:
         BUG_ON(!b->hold_count);
         BUG_ON(test_bit(B_READING, &b->state));
 
-        __write_dirty_buffer(b);
+        __write_dirty_buffer(b, NULL);
         if (b->hold_count == 1) {
                 wait_on_bit(&b->state, B_WRITING,
                             do_io_schedule, TASK_UNINTERRUPTIBLE);