drbd: improve throttling decisions of background resynchronisation

Background resynchronisation does some "side-stepping", or throttles
itself, if it detects application IO activity, and the current resync
rate estimate is above the configured "cmin-rate".

What was not detected: if there is no application IO,
because it blocks on activity log transactions.

Introduce a new atomic_t ap_actlog_cnt, tracking such blocked requests,
and count non-zero as application IO activity.
This counter is exposed at proc_details level 2 and above.

Also make sure to release the currently locked resync extent
if we side-step due to such voluntary throttling.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

7 files changed, 51 insertions, 18 deletions

diff --git a/drivers/block/drbd/drbd_actlog.c b/drivers/block/drbd/drbd_actlog.c
index d7e80663187c..6ce5c76d642b 100644
--- a/drivers/block/drbd/drbd_actlog.c
+++ b/drivers/block/drbd/drbd_actlog.c
@@ -991,6 +991,15 @@ int drbd_try_rs_begin_io(struct drbd_device *device, sector_t sector)
         struct lc_element *e;
         struct bm_extent *bm_ext;
         int i;
+        bool throttle = drbd_rs_should_slow_down(device, sector, true);
+
+        /* If we need to throttle, a half-locked (only marked BME_NO_WRITES,
+         * not yet BME_LOCKED) extent needs to be kicked out explicitly if we
+         * need to throttle. There is at most one such half-locked extent,
+         * which is remembered in resync_wenr. */
+
+        if (throttle && device->resync_wenr != enr)
+                return -EAGAIN;
 
         spin_lock_irq(&device->al_lock);
         if (device->resync_wenr != LC_FREE && device->resync_wenr != enr) {
@@ -1014,8 +1023,10 @@ int drbd_try_rs_begin_io(struct drbd_device *device, sector_t sector)
                         D_ASSERT(device, test_bit(BME_NO_WRITES, &bm_ext->flags));
                         clear_bit(BME_NO_WRITES, &bm_ext->flags);
                         device->resync_wenr = LC_FREE;
-                        if (lc_put(device->resync, &bm_ext->lce) == 0)
+                        if (lc_put(device->resync, &bm_ext->lce) == 0) {
+                                bm_ext->flags = 0;
                                 device->resync_locked--;
+                        }
                         wake_up(&device->al_wait);
                 } else {
                         drbd_alert(device, "LOGIC BUG\n");
@@ -1077,8 +1088,20 @@ proceed:
         return 0;
 
 try_again:
-        if (bm_ext)
-                device->resync_wenr = enr;
+        if (bm_ext) {
+                if (throttle) {
+                        D_ASSERT(device, !test_bit(BME_LOCKED, &bm_ext->flags));
+                        D_ASSERT(device, test_bit(BME_NO_WRITES, &bm_ext->flags));
+                        clear_bit(BME_NO_WRITES, &bm_ext->flags);
+                        device->resync_wenr = LC_FREE;
+                        if (lc_put(device->resync, &bm_ext->lce) == 0) {
+                                bm_ext->flags = 0;
+                                device->resync_locked--;
+                        }
+                        wake_up(&device->al_wait);
+                } else
+                        device->resync_wenr = enr;
+        }
         spin_unlock_irq(&device->al_lock);
         return -EAGAIN;
 }
diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index fa010ea3a4bf..81f4af49b8ac 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -797,6 +797,7 @@ struct drbd_device {
         unsigned int al_writ_cnt;
         unsigned int bm_writ_cnt;
         atomic_t ap_bio_cnt;     /* Requests we need to complete */
+        atomic_t ap_actlog_cnt;  /* Requests waiting for activity log */
         atomic_t ap_pending_cnt; /* AP data packets on the wire, ack expected */
         atomic_t rs_pending_cnt; /* RS request/data packets on the wire */
         atomic_t unacked_cnt;    /* Need to send replies for */
@@ -1454,7 +1455,8 @@ extern void drbd_endio_write_sec_final(struct drbd_peer_request *peer_req);
 extern int drbd_receiver(struct drbd_thread *thi);
 extern int drbd_asender(struct drbd_thread *thi);
 extern bool drbd_rs_c_min_rate_throttle(struct drbd_device *device);
-extern bool drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector);
+extern bool drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector,
+                bool throttle_if_app_is_waiting);
 extern int drbd_submit_peer_request(struct drbd_device *,
                                     struct drbd_peer_request *, const unsigned,
                                     const int);
diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index 58865969c9f4..ad7c0e8843c4 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -1909,6 +1909,7 @@ void drbd_init_set_defaults(struct drbd_device *device)
         drbd_set_defaults(device);
 
         atomic_set(&device->ap_bio_cnt, 0);
+        atomic_set(&device->ap_actlog_cnt, 0);
         atomic_set(&device->ap_pending_cnt, 0);
         atomic_set(&device->rs_pending_cnt, 0);
         atomic_set(&device->unacked_cnt, 0);
diff --git a/drivers/block/drbd/drbd_proc.c b/drivers/block/drbd/drbd_proc.c
index 9059d7bf8a36..06e6147c7601 100644
--- a/drivers/block/drbd/drbd_proc.c
+++ b/drivers/block/drbd/drbd_proc.c
@@ -335,6 +335,9 @@ static int drbd_seq_show(struct seq_file *seq, void *v)
                         lc_seq_printf_stats(seq, device->act_log);
                         put_ldev(device);
                 }
+
+                if (proc_details >= 2)
+                        seq_printf(seq, "\tblocked on activity log: %d\n", atomic_read(&device->ap_actlog_cnt));
         }
         rcu_read_unlock();
 
diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 7a1078d285dd..0d3cbd8e4b9c 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -2417,13 +2417,14 @@ out_interrupted:
  * The current sync rate used here uses only the most recent two step marks,
  * to have a short time average so we can react faster.
  */
-bool drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector)
+bool drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector,
+                bool throttle_if_app_is_waiting)
 {
         struct lc_element *tmp;
-        bool throttle = true;
+        bool throttle = drbd_rs_c_min_rate_throttle(device);
 
-        if (!drbd_rs_c_min_rate_throttle(device))
-                return false;
+        if (!throttle || throttle_if_app_is_waiting)
+                return throttle;
 
         spin_lock_irq(&device->al_lock);
         tmp = lc_find(device->resync, BM_SECT_TO_EXT(sector));
@@ -2431,7 +2432,8 @@ bool drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector)
                 struct bm_extent *bm_ext = lc_entry(tmp, struct bm_extent, lce);
                 if (test_bit(BME_PRIORITY, &bm_ext->flags))
                         throttle = false;
-                /* Do not slow down if app IO is already waiting for this extent */
+                /* Do not slow down if app IO is already waiting for this extent,
+                 * and our progress is necessary for application IO to complete. */
         }
         spin_unlock_irq(&device->al_lock);
 
@@ -2456,7 +2458,9 @@ bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
         curr_events = (int)part_stat_read(&disk->part0, sectors[0]) +
                       (int)part_stat_read(&disk->part0, sectors[1]) -
                         atomic_read(&device->rs_sect_ev);
-        if (!device->rs_last_events || curr_events - device->rs_last_events > 64) {
+
+        if (atomic_read(&device->ap_actlog_cnt)
+            || !device->rs_last_events || curr_events - device->rs_last_events > 64) {
                 unsigned long rs_left;
                 int i;
 
@@ -2646,7 +2650,8 @@ static int receive_DataRequest(struct drbd_connection *connection, struct packet
          * we would also throttle its application reads.
          * In that case, throttling is done on the SyncTarget only.
          */
-        if (device->state.peer != R_PRIMARY && drbd_rs_should_slow_down(device, sector))
+        if (device->state.peer != R_PRIMARY
+        && drbd_rs_should_slow_down(device, sector, false))
                 schedule_timeout_uninterruptible(HZ/10);
         if (drbd_rs_begin_io(device, sector))
                 goto out_free_e;
diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
index 3f6a6ed2fd03..74ebef101dc7 100644
--- a/drivers/block/drbd/drbd_req.c
+++ b/drivers/block/drbd/drbd_req.c
@@ -1218,6 +1218,7 @@ drbd_request_prepare(struct drbd_device *device, struct bio *bio, unsigned long
         if (rw == WRITE && req->private_bio && req->i.size
         && !test_bit(AL_SUSPENDED, &device->flags)) {
                 if (!drbd_al_begin_io_fastpath(device, &req->i)) {
+                        atomic_inc(&device->ap_actlog_cnt);
                         drbd_queue_write(device, req);
                         return NULL;
                 }
@@ -1354,6 +1355,7 @@ static void submit_fast_path(struct drbd_device *device, struct list_head *incom
 
                         req->rq_state |= RQ_IN_ACT_LOG;
                         req->in_actlog_jif = jiffies;
+                        atomic_dec(&device->ap_actlog_cnt);
                 }
 
                 list_del_init(&req->tl_requests);
@@ -1439,6 +1441,7 @@ skip_fast_path:
                 list_for_each_entry_safe(req, tmp, &pending, tl_requests) {
                         req->rq_state |= RQ_IN_ACT_LOG;
                         req->in_actlog_jif = jiffies;
+                        atomic_dec(&device->ap_actlog_cnt);
                         list_del_init(&req->tl_requests);
                         drbd_send_and_submit(device, req);
                 }
@@ -1454,6 +1457,7 @@ skip_fast_path:
                         if (!was_cold) {
                                 req->rq_state |= RQ_IN_ACT_LOG;
                                 req->in_actlog_jif = jiffies;
+                                atomic_dec(&device->ap_actlog_cnt);
                                 /* Corresponding extent was hot after all? */
                                 drbd_send_and_submit(device, req);
                         } else {
diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c
index 0ff8f4637741..48975a264985 100644
--- a/drivers/block/drbd/drbd_worker.c
+++ b/drivers/block/drbd/drbd_worker.c
@@ -395,9 +395,6 @@ static int read_for_csum(struct drbd_peer_device *peer_device, sector_t sector,
         if (!get_ldev(device))
                 return -EIO;
 
-        if (drbd_rs_should_slow_down(device, sector))
-                goto defer;
-
         /* GFP_TRY, because if there is no memory available right now, this may
          * be rescheduled for later. It is "only" background resync, after all. */
         peer_req = drbd_alloc_peer_req(peer_device, ID_SYNCER /* unused */, sector,
@@ -651,8 +648,7 @@ next_sector:
 
                 sector = BM_BIT_TO_SECT(bit);
 
-                if (drbd_rs_should_slow_down(device, sector) ||
-                    drbd_try_rs_begin_io(device, sector)) {
+                if (drbd_try_rs_begin_io(device, sector)) {
                         device->bm_resync_fo = bit;
                         goto requeue;
                 }
@@ -783,8 +779,7 @@ static int make_ov_request(struct drbd_device *device, int cancel)
 
                 size = BM_BLOCK_SIZE;
 
-                if (drbd_rs_should_slow_down(device, sector) ||
-                    drbd_try_rs_begin_io(device, sector)) {
+                if (drbd_try_rs_begin_io(device, sector)) {
                         device->ov_position = sector;
                         goto requeue;
                 }