The software iscsi driver's queuecommand can block and taking the extra
hop from kblockd to its workqueue results in a performance hit. Allowing
it to set BLK_MQ_F_BLOCKING and transmit from that context directly
results in a 20-30% improvement in IOPs for workloads like:
fio --filename=/dev/sdb --direct=1 --rw=randrw --bs=4k --ioengine=libaio
--iodepth=128 --numjobs=1
and for all write workloads.
when using the none scheduler and the app and iscsi bound to the same
CPUs. Throughput tests show similar gains.
This patch adds a new scsi_host_template field so drivers can tell scsi-ml
that they can block so scsi-ml can setup the tag set with the
BLK_MQ_F_BLOCKING flag.
Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
---
drivers/scsi/scsi_lib.c | 6 ++++--
include/scsi/scsi_host.h | 4 ++++
2 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0a70aa763a96..a5dbeb9994ae 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1989,6 +1989,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
tag_set->driver_data = shost;
if (shost->host_tagset)
tag_set->flags |= BLK_MQ_F_TAG_HCTX_SHARED;
+ if (shost->hostt->queuecommand_blocks)
+ tag_set->flags |= BLK_MQ_F_BLOCKING;
return blk_mq_alloc_tag_set(tag_set);
}
@@ -2952,8 +2954,8 @@ scsi_host_block(struct Scsi_Host *shost)
}
/*
- * SCSI never enables blk-mq's BLK_MQ_F_BLOCKING flag so
- * calling synchronize_rcu() once is enough.
+ * Drivers that use this helper enable blk-mq's BLK_MQ_F_BLOCKING flag
+ * so calling synchronize_rcu() once is enough.
*/
WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 72e1a347baa6..0d106dc9309d 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -75,6 +75,10 @@ struct scsi_host_template {
*/
int (* queuecommand)(struct Scsi_Host *, struct scsi_cmnd *);
+ /*
+ * Set To true if the queuecommand function can block.
+ */
+ bool queuecommand_blocks;
/*
* The commit_rqs function is used to trigger a hardware
* doorbell after some requests have been queued with
--
2.25.1
