[PATCH 3/3] scsi: avoid to hold host-wide counter of host_busy for scsi_mq

Ming Lei
2018-04-20T06:59:48Z

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
level, so we should avoid to this expensive atomic operations on the
hostwide counter in IO path.

This patch implemens 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 one
dual-socket system.

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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
drivers/scsi/hosts.c | 24 +++++++++++++++++++++++-
drivers/scsi/scsi_lib.c | 23 +++++++++++++++++------
2 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 69beb30205f1..ad56e2b10ac8 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -564,13 +564,35 @@ 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 0dfec0dedd5e..dc437c642934 100644
--- 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)
@@ -1539,9 +1545,12 @@ 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;

/*
@@ -1555,7 +1564,7 @@ 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;
@@ -1641,7 +1650,9 @@ 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);

--
2.9.5