Hi Tang Junhui---

On 12/28/2017 06:00 PM, tang.junhui@zte.com.cn wrote:

LGTM, I would like it much more if MAX_WRITEBACKS_IN_PASS(5) defines a 
little bigger value.

Yes-- we can always turn it up further depending on experience.  5 was
chosen to provide a clear benefit without changing things too much.

Reviewed-by: Tang Junhui <redacted>

Thank you very much.  Applied to my staging tree.

Mike

quoted

Previously, there was some logic that attempted to immediately issue
writeback of backing-contiguous blocks when the writeback rate was
fast.

The previous logic did not have any limits on the aggregate size it
would issue, nor the number of keys it would combine at once.  It
would also discard the chance to do a contiguous write when the
writeback rate was low-- e.g. at "background" writeback of target
rate = 8, it would not combine two adjacent 4k writes and would
instead seek the disk twice.

This patch imposes limits and explicitly understands the size of
contiguous I/O during issue.  It also will combine contiguous I/O
in all circumstances, not just when writeback is requested to be
relatively fast.

It is a win on its own, but also lays the groundwork for skip writes to
short keys to make the I/O more sequential/contiguous.  It also gets
ready to start using blk_*_plug, and to allow issuing of non-contig
I/O in parallel if requested by the user (to make use of disk
throughput benefits available from higher queue depths).

This patch fixes a previous version where the contiguous information
was not calculated properly, thanks to suggestions by Tang Junhui.

Signed-off-by: Michael Lyle <redacted>
---
drivers/md/bcache/bcache.h    |   6 ---
drivers/md/bcache/writeback.c | 120 +++++++++++++++++++++++++++++-------------
drivers/md/bcache/writeback.h |   3 ++
3 files changed, 85 insertions(+), 44 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 843877e017e1..1784e50eb857 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h

@@ -323,12 +323,6 @@ struct cached_dev {
    struct bch_ratelimit    writeback_rate;
    struct delayed_work    writeback_rate_update;

-    /*
-     * Internal to the writeback code, so read_dirty() can keep track of
-     * where it's at.
-     */
-    sector_t        last_read;
-
    /* Limit number of writeback bios in flight */
    struct semaphore    in_flight;
    struct task_struct    *writeback_thread;

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index f3d680c907ae..6975003454d9 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c

@@ -251,7 +251,9 @@ static void read_dirty_submit(struct closure *cl)

static void read_dirty(struct cached_dev *dc)
{
    unsigned delay = 0;
-    struct keybuf_key *w;
+    struct keybuf_key *next, *keys[MAX_WRITEBACKS_IN_PASS], *w;
+    size_t size;
+    int nk, i;
    struct dirty_io *io;
    struct closure cl;

@@ -262,45 +264,87 @@ static void read_dirty(struct cached_dev *dc)
     * mempools.
     */

-    while (!kthread_should_stop()) {
-
-        w = bch_keybuf_next(&dc->writeback_keys);
-        if (!w)
-            break;
-
-        BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
-
-        if (KEY_START(&w->key) != dc->last_read ||
-            jiffies_to_msecs(delay) > 50)
-            while (!kthread_should_stop() && delay)
-                delay = schedule_timeout_interruptible(delay);
-
-        dc->last_read    = KEY_OFFSET(&w->key);
-
-        io = kzalloc(sizeof(struct dirty_io) + sizeof(struct bio_vec)
-                 * DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
-                 GFP_KERNEL);
-        if (!io)
-            goto err;
-
-        w->private    = io;
-        io->dc        = dc;
-
-        dirty_init(w);
-        bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
-        io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
-        bio_set_dev(&io->bio, PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
-        io->bio.bi_end_io    = read_dirty_endio;
-
-        if (bio_alloc_pages(&io->bio, GFP_KERNEL))
-            goto err_free;
-
-        trace_bcache_writeback(&w->key);
+    next = bch_keybuf_next(&dc->writeback_keys);
+
+    while (!kthread_should_stop() && next) {
+        size = 0;
+        nk = 0;
+
+        do {
+            BUG_ON(ptr_stale(dc->disk.c, &next->key, 0));
+
+            /*
+             * Don't combine too many operations, even if they
+             * are all small.
+             */
+            if (nk >= MAX_WRITEBACKS_IN_PASS)
+                break;
+
+            /*
+             * If the current operation is very large, don't
+             * further combine operations.
+             */
+            if (size >= MAX_WRITESIZE_IN_PASS)
+                break;
+
+            /*
+             * Operations are only eligible to be combined
+             * if they are contiguous.
+             *
+             * TODO: add a heuristic willing to fire a
+             * certain amount of non-contiguous IO per pass,
+             * so that we can benefit from backing device
+             * command queueing.
+             */
+            if ((nk != 0) && bkey_cmp(&keys[nk-1]->key,
+                        &START_KEY(&next->key)))
+                break;
+
+            size += KEY_SIZE(&next->key);
+            keys[nk++] = next;
+        } while ((next = bch_keybuf_next(&dc->writeback_keys)));
+
+        /* Now we have gathered a set of 1..5 keys to write back. */
+        for (i = 0; i < nk; i++) {
+            w = keys[i];
+
+            io = kzalloc(sizeof(struct dirty_io) +
+                     sizeof(struct bio_vec) *
+                     DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
+                     GFP_KERNEL);
+            if (!io)
+                goto err;
+
+            w->private    = io;
+            io->dc        = dc;
+
+            dirty_init(w);
+            bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
+            io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
+            bio_set_dev(&io->bio,
+                    PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
+            io->bio.bi_end_io    = read_dirty_endio;
+
+            if (bio_alloc_pages(&io->bio, GFP_KERNEL))
+                goto err_free;
+
+            trace_bcache_writeback(&w->key);
+
+            down(&dc->in_flight);
+
+            /* We've acquired a semaphore for the maximum
+             * simultaneous number of writebacks; from here
+             * everything happens asynchronously.
+             */
+            closure_call(&io->cl, read_dirty_submit, NULL, &cl);
+        }

-        down(&dc->in_flight);
-        closure_call(&io->cl, read_dirty_submit, NULL, &cl);
+        delay = writeback_delay(dc, size);

-        delay = writeback_delay(dc, KEY_SIZE(&w->key));
+        while (!kthread_should_stop() && delay) {
+            schedule_timeout_interruptible(delay);
+            delay = writeback_delay(dc, 0);
+        }
    }

    if (0) {

diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index a9e3ffb4b03c..6d26927267f8 100644
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h

@@ -5,6 +5,9 @@

#define CUTOFF_WRITEBACK    40
#define CUTOFF_WRITEBACK_SYNC    70

+#define MAX_WRITEBACKS_IN_PASS  5
+#define MAX_WRITESIZE_IN_PASS   5000    /* *512b */
+
static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device *d)
{
    uint64_t i, ret = 0;
-- 
2.14.1

Thanks,
Tang


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