drbd: don't let application IO pre-empt resync too often

Before, application IO could pre-empt resync activity
for up to hardcoded 20 seconds per resync request.
A very busy server could throttle the effective resync bandwidth
down to one request per 20 seconds.

Now, we only let application IO pre-empt resync traffic
while the current resync rate estimate is above c-min-rate.

If you disable the c-min-rate throttle feature (set c-min-rate = 0),
application IO will no longer pre-empt resync traffic at all.

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

3 files changed, 34 insertions, 29 deletions

diff --git a/drivers/block/drbd/drbd_actlog.c b/drivers/block/drbd/drbd_actlog.c
index 7e7b0e143655..8dd09a7f23c6 100644
--- a/drivers/block/drbd/drbd_actlog.c
+++ b/drivers/block/drbd/drbd_actlog.c
@@ -1022,8 +1022,7 @@ int drbd_rs_begin_io(struct drbd_device *device, sector_t sector)
         unsigned int enr = BM_SECT_TO_EXT(sector);
         struct bm_extent *bm_ext;
         int i, sig;
-        int sa = 200; /* Step aside 200 times, then grab the extent and let app-IO wait.
-                         200 times -> 20 seconds. */
+        bool sa;
 
 retry:
         sig = wait_event_interruptible(device->al_wait,
@@ -1034,12 +1033,15 @@ retry:
         if (test_bit(BME_LOCKED, &bm_ext->flags))
                 return 0;
 
+        /* step aside only while we are above c-min-rate; unless disabled. */
+        sa = drbd_rs_c_min_rate_throttle(device);
+
         for (i = 0; i < AL_EXT_PER_BM_SECT; i++) {
                 sig = wait_event_interruptible(device->al_wait,
                                                !_is_in_al(device, enr * AL_EXT_PER_BM_SECT + i) ||
-                                               test_bit(BME_PRIORITY, &bm_ext->flags));
+                                               (sa && test_bit(BME_PRIORITY, &bm_ext->flags)));
 
-                if (sig || (test_bit(BME_PRIORITY, &bm_ext->flags) && sa)) {
+                if (sig || (sa && test_bit(BME_PRIORITY, &bm_ext->flags))) {
                         spin_lock_irq(&device->al_lock);
                         if (lc_put(device->resync, &bm_ext->lce) == 0) {
                                 bm_ext->flags = 0; /* clears BME_NO_WRITES and eventually BME_PRIORITY */
@@ -1051,9 +1053,6 @@ retry:
                                 return -EINTR;
                         if (schedule_timeout_interruptible(HZ/10))
                                 return -EINTR;
-                        if (sa && --sa == 0)
-                                drbd_warn(device, "drbd_rs_begin_io() stepped aside for 20sec."
-                                         "Resync stalled?\n");
                         goto retry;
                 }
         }
diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index 361a2e9cd727..f0cabea5cda2 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -1339,7 +1339,8 @@ extern void start_resync_timer_fn(unsigned long data);
 /* drbd_receiver.c */
 extern int drbd_receiver(struct drbd_thread *thi);
 extern int drbd_asender(struct drbd_thread *thi);
-extern int drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector);
+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 int drbd_submit_peer_request(struct drbd_device *,
                                     struct drbd_peer_request *, const unsigned,
                                     const int);
diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 6ffbc22eba0b..10d2dcb16bff 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -2323,39 +2323,45 @@ 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.
  */
-int 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)
 {
-        struct gendisk *disk = device->ldev->backing_bdev->bd_contains->bd_disk;
-        unsigned long db, dt, dbdt;
         struct lc_element *tmp;
-        int curr_events;
-        int throttle = 0;
-        unsigned int c_min_rate;
+        bool throttle = true;
 
-        rcu_read_lock();
-        c_min_rate = rcu_dereference(device->ldev->disk_conf)->c_min_rate;
-        rcu_read_unlock();
-
-        /* feature disabled? */
-        if (c_min_rate == 0)
-                return 0;
+        if (!drbd_rs_c_min_rate_throttle(device))
+                return false;
 
         spin_lock_irq(&device->al_lock);
         tmp = lc_find(device->resync, BM_SECT_TO_EXT(sector));
         if (tmp) {
                 struct bm_extent *bm_ext = lc_entry(tmp, struct bm_extent, lce);
-                if (test_bit(BME_PRIORITY, &bm_ext->flags)) {
-                        spin_unlock_irq(&device->al_lock);
-                        return 0;
-                }
+                if (test_bit(BME_PRIORITY, &bm_ext->flags))
+                        throttle = false;
                 /* Do not slow down if app IO is already waiting for this extent */
         }
         spin_unlock_irq(&device->al_lock);
 
+        return throttle;
+}
+
+bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
+{
+        struct gendisk *disk = device->ldev->backing_bdev->bd_contains->bd_disk;
+        unsigned long db, dt, dbdt;
+        unsigned int c_min_rate;
+        int curr_events;
+
+        rcu_read_lock();
+        c_min_rate = rcu_dereference(device->ldev->disk_conf)->c_min_rate;
+        rcu_read_unlock();
+
+        /* feature disabled? */
+        if (c_min_rate == 0)
+                return false;
+
         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) {
                 unsigned long rs_left;
                 int i;
@@ -2378,12 +2384,11 @@ int drbd_rs_should_slow_down(struct drbd_device *device, sector_t sector)
                 dbdt = Bit2KB(db/dt);
 
                 if (dbdt > c_min_rate)
-                        throttle = 1;
+                        return true;
         }
-        return throttle;
+        return false;
 }
 
-
 static int receive_DataRequest(struct drbd_connection *connection, struct packet_info *pi)
 {
         struct drbd_peer_device *peer_device;