drbd: Load balancing of read requests

New config option for the disk secition "read-balancing", with
the values: prefer-local, prefer-remote, round-robin, when-congested-remote.

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

6 files changed, 68 insertions, 2 deletions

diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index d397681fb7aa..e2cccb40f5af 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -698,6 +698,7 @@ enum {
         AHEAD_TO_SYNC_SOURCE,   /* Ahead -> SyncSource queued */
         B_RS_H_DONE,            /* Before resync handler done (already executed) */
         DISCARD_MY_DATA,        /* discard_my_data flag per volume */
+        READ_BALANCE_RR,
 };
 
 struct drbd_bitmap; /* opaque for drbd_conf */
diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index e546dd3fab8a..733b8bd663d5 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -4974,7 +4974,7 @@ static int got_NegDReply(struct drbd_tconn *tconn, struct packet_info *pi)
 
         update_peer_seq(mdev, be32_to_cpu(p->seq_num));
 
-        dev_err(DEV, "Got NegDReply; Sector %llus, len %u; Fail original request.\n",
+        dev_err(DEV, "Got NegDReply; Sector %llus, len %u.\n",
             (unsigned long long)sector, be32_to_cpu(p->blksize));
 
         return validate_req_change_req_state(mdev, p->block_id, sector,
diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
index ceb04a94aace..98251e2a7fb7 100644
--- a/drivers/block/drbd/drbd_req.c
+++ b/drivers/block/drbd/drbd_req.c
@@ -563,6 +563,11 @@ int __req_mod(struct drbd_request *req, enum drbd_req_event what,
                 if (req->rq_state & RQ_NET_SENT && req->rq_state & RQ_WRITE)
                         atomic_sub(req->i.size >> 9, &mdev->ap_in_flight);
 
+                if (!(req->rq_state & RQ_WRITE) &&
+                    mdev->state.disk == D_UP_TO_DATE &&
+                    !IS_ERR_OR_NULL(req->private_bio))
+                        goto goto_read_retry_local;
+
                 /* if it is still queued, we may not complete it here.
                  * it will be canceled soon. */
                 if (!(req->rq_state & RQ_NET_QUEUED))
@@ -625,10 +630,22 @@ int __req_mod(struct drbd_request *req, enum drbd_req_event what,
                 req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);
 
                 req->rq_state |= RQ_NET_DONE;
+
+                if (!(req->rq_state & RQ_WRITE) &&
+                    mdev->state.disk == D_UP_TO_DATE &&
+                    !IS_ERR_OR_NULL(req->private_bio))
+                        goto goto_read_retry_local;
+
                 _req_may_be_done_not_susp(req, m);
                 /* else: done by HANDED_OVER_TO_NETWORK */
                 break;
 
+        goto_read_retry_local:
+                req->rq_state |= RQ_LOCAL_PENDING;
+                req->private_bio->bi_bdev = mdev->ldev->backing_bdev;
+                generic_make_request(req->private_bio);
+                break;
+
         case FAIL_FROZEN_DISK_IO:
                 if (!(req->rq_state & RQ_LOCAL_COMPLETED))
                         break;
@@ -689,6 +706,11 @@ int __req_mod(struct drbd_request *req, enum drbd_req_event what,
                 dec_ap_pending(mdev);
                 req->rq_state &= ~RQ_NET_PENDING;
                 req->rq_state |= (RQ_NET_OK|RQ_NET_DONE);
+                if (!IS_ERR_OR_NULL(req->private_bio)) {
+                        bio_put(req->private_bio);
+                        req->private_bio = NULL;
+                        put_ldev(mdev);
+                }
                 _req_may_be_done_not_susp(req, m);
                 break;
         };
@@ -723,6 +745,35 @@ static bool drbd_may_do_local_read(struct drbd_conf *mdev, sector_t sector, int
         return drbd_bm_count_bits(mdev, sbnr, ebnr) == 0;
 }
 
+static bool remote_due_to_read_balancing(struct drbd_conf *mdev)
+{
+        enum drbd_read_balancing rbm;
+        struct backing_dev_info *bdi;
+
+        if (mdev->state.pdsk < D_UP_TO_DATE)
+                return false;
+
+        rcu_read_lock();
+        rbm = rcu_dereference(mdev->ldev->disk_conf)->read_balancing;
+        rcu_read_unlock();
+
+        switch (rbm) {
+        case RB_CONGESTED_REMOTE:
+                bdi = &mdev->ldev->backing_bdev->bd_disk->queue->backing_dev_info;
+                return bdi_read_congested(bdi);
+        case RB_LEAST_PENDING:
+                return atomic_read(&mdev->local_cnt) >
+                        atomic_read(&mdev->ap_pending_cnt) + atomic_read(&mdev->rs_pending_cnt);
+        case RB_ROUND_ROBIN:
+                return test_and_change_bit(READ_BALANCE_RR, &mdev->flags);
+        case RB_PREFER_REMOTE:
+                return true;
+        case RB_PREFER_LOCAL:
+        default:
+                return false;
+        }
+}
+
 /*
  * complete_conflicting_writes  -  wait for any conflicting write requests
  *
@@ -790,6 +841,10 @@ int __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long s
                                 bio_put(req->private_bio);
                                 req->private_bio = NULL;
                                 put_ldev(mdev);
+                        } else if (remote_due_to_read_balancing(mdev)) {
+                                /* Keep the private bio in case we need it
+                                   for a local retry */
+                                local = 0;
                         }
                 }
                 remote = !local && mdev->state.pdsk >= D_UP_TO_DATE;
@@ -1017,7 +1072,7 @@ fail_free_complete:
         if (req->rq_state & RQ_IN_ACT_LOG)
                 drbd_al_complete_io(mdev, &req->i);
 fail_and_free_req:
-        if (local) {
+        if (!IS_ERR_OR_NULL(req->private_bio)) {
                 bio_put(req->private_bio);
                 req->private_bio = NULL;
                 put_ldev(mdev);
diff --git a/include/linux/drbd.h b/include/linux/drbd.h
index 1e9f754b66ac..157ba3d74dc7 100644
--- a/include/linux/drbd.h
+++ b/include/linux/drbd.h
@@ -102,6 +102,14 @@ enum drbd_on_congestion {
         OC_DISCONNECT,
 };
 
+enum drbd_read_balancing {
+        RB_PREFER_LOCAL,
+        RB_PREFER_REMOTE,
+        RB_ROUND_ROBIN,
+        RB_LEAST_PENDING,
+        RB_CONGESTED_REMOTE,
+};
+
 /* KEEP the order, do not delete or insert. Only append. */
 enum drbd_ret_code {
         ERR_CODE_BASE           = 100,
diff --git a/include/linux/drbd_genl.h b/include/linux/drbd_genl.h
index 2e6cefefe5e5..826008f297fe 100644
--- a/include/linux/drbd_genl.h
+++ b/include/linux/drbd_genl.h
@@ -129,6 +129,7 @@ GENL_struct(DRBD_NLA_DISK_CONF, 3, disk_conf,
         __flg_field_def(18, DRBD_GENLA_F_MANDATORY,     disk_drain, DRBD_DISK_DRAIN_DEF)
         __flg_field_def(19, DRBD_GENLA_F_MANDATORY,     md_flushes, DRBD_MD_FLUSHES_DEF)
         __u32_field_def(20,     DRBD_GENLA_F_MANDATORY, disk_timeout, DRBD_DISK_TIMEOUT_DEF)
+        __u32_field_def(21,     0 /* OPTIONAL */,       read_balancing, DRBD_READ_BALANCING_DEF)
 )
 
 GENL_struct(DRBD_NLA_RESOURCE_OPTS, 4, res_opts,
diff --git a/include/linux/drbd_limits.h b/include/linux/drbd_limits.h
index 6d0a24331ed2..17ef66a5c114 100644
--- a/include/linux/drbd_limits.h
+++ b/include/linux/drbd_limits.h
@@ -161,6 +161,7 @@
 #define DRBD_RR_CONFLICT_DEF ASB_DISCONNECT
 #define DRBD_ON_NO_DATA_DEF OND_IO_ERROR
 #define DRBD_ON_CONGESTION_DEF OC_BLOCK
+#define DRBD_READ_BALANCING_DEF RB_PREFER_LOCAL
 
 #define DRBD_MAX_BIO_BVECS_MIN 0
 #define DRBD_MAX_BIO_BVECS_MAX 128