scsi: core: avoid host-wide host_busy counter for scsi_mq

It isn't necessary to check the host depth in scsi_queue_rq() any more
since it has been respected by blk-mq before calling scsi_queue_rq() via
getting driver tag.

Lots of LUNs may attach to same host and per-host IOPS may reach millions,
so we should avoid expensive atomic operations on the host-wide counter in
the IO path.

This patch implements scsi_host_busy() via blk_mq_tagset_busy_iter() for
reading the count of busy IOs for scsi_mq.

It is observed that IOPS is increased by 15% in IO test on scsi_debug (32
LUNs, 32 submit queues, 1024 can_queue, libaio/dio) in a dual-socket
system.

[mkp: clarified commit message]

Cc: Omar Sandoval <osandov@fb.com>,
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
Cc: James Bottomley <james.bottomley@hansenpartnership.com>,
Cc: Christoph Hellwig <hch@lst.de>,
Cc: Don Brace <don.brace@microsemi.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index ea4b0bb0c1cd4e6b12748256741afae339311591..f02dcc875a09a6c6ece63e89e8256c512e11f01e 100644 (file)
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -563,13 +563,35 @@ struct Scsi_Host *scsi_host_get(struct Scsi_Host *shost)
 }
 EXPORT_SYMBOL(scsi_host_get);
 
+struct scsi_host_mq_in_flight {
+       int cnt;
+};
+
+static void scsi_host_check_in_flight(struct request *rq, void *data,
+               bool reserved)
+{
+       struct scsi_host_mq_in_flight *in_flight = data;
+
+       if (blk_mq_request_started(rq))
+               in_flight->cnt++;
+}
+
 /**
  * scsi_host_busy - Return the host busy counter
  * @shost:     Pointer to Scsi_Host to inc.
  **/
 int scsi_host_busy(struct Scsi_Host *shost)
 {
-       return atomic_read(&shost->host_busy);
+       struct scsi_host_mq_in_flight in_flight = {
+               .cnt = 0,
+       };
+
+       if (!shost->use_blk_mq)
+               return atomic_read(&shost->host_busy);
+
+       blk_mq_tagset_busy_iter(&shost->tag_set, scsi_host_check_in_flight,
+                       &in_flight);
+       return in_flight.cnt;
 }
 EXPORT_SYMBOL(scsi_host_busy);
 

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index a1a0a2903263c4c4d85cc0760fd8fe9309d2c0d1..600c78065d6234afd3bf93c8b2c67f1961ec0432 100644 (file)
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -345,7 +345,8 @@ static void scsi_dec_host_busy(struct Scsi_Host *shost)
        unsigned long flags;
 
        rcu_read_lock();
-       atomic_dec(&shost->host_busy);
+       if (!shost->use_blk_mq)
+               atomic_dec(&shost->host_busy);
        if (unlikely(scsi_host_in_recovery(shost))) {
                spin_lock_irqsave(shost->host_lock, flags);
                if (shost->host_failed || shost->host_eh_scheduled)
@@ -444,7 +445,12 @@ static inline bool scsi_target_is_busy(struct scsi_target *starget)
 
 static inline bool scsi_host_is_busy(struct Scsi_Host *shost)
 {
-       if (shost->can_queue > 0 &&
+       /*
+        * blk-mq can handle host queue busy efficiently via host-wide driver
+        * tag allocation
+        */
+
+       if (!shost->use_blk_mq && shost->can_queue > 0 &&
            atomic_read(&shost->host_busy) >= shost->can_queue)
                return true;
        if (atomic_read(&shost->host_blocked) > 0)
@@ -1600,9 +1606,12 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
        if (scsi_host_in_recovery(shost))
                return 0;
 
-       busy = atomic_inc_return(&shost->host_busy) - 1;
+       if (!shost->use_blk_mq)
+               busy = atomic_inc_return(&shost->host_busy) - 1;
+       else
+               busy = 0;
        if (atomic_read(&shost->host_blocked) > 0) {
-               if (busy)
+               if (busy || scsi_host_busy(shost))
                        goto starved;
 
                /*
@@ -1616,7 +1625,7 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
                                     "unblocking host at zero depth\n"));
        }
 
-       if (shost->can_queue > 0 && busy >= shost->can_queue)
+       if (!shost->use_blk_mq && shost->can_queue > 0 && busy >= shost->can_queue)
                goto starved;
        if (shost->host_self_blocked)
                goto starved;
@@ -1702,7 +1711,9 @@ static void scsi_kill_request(struct request *req, struct request_queue *q)
         * with the locks as normal issue path does.
         */
        atomic_inc(&sdev->device_busy);
-       atomic_inc(&shost->host_busy);
+
+       if (!shost->use_blk_mq)
+               atomic_inc(&shost->host_busy);
        if (starget->can_queue > 0)
                atomic_inc(&starget->target_busy);