scsi: add support for a blk-mq based I/O path.
This patch adds support for an alternate I/O path in the scsi midlayer
which uses the blk-mq infrastructure instead of the legacy request code.
Use of blk-mq is fully transparent to drivers, although for now a host
template field is provided to opt out of blk-mq usage in case any unforseen
incompatibilities arise.
In general replacing the legacy request code with blk-mq is a simple and
mostly mechanical transformation. The biggest exception is the new code
that deals with the fact the I/O submissions in blk-mq must happen from
process context, which slightly complicates the I/O completion handler.
The second biggest differences is that blk-mq is build around the concept
of preallocated requests that also include driver specific data, which
in SCSI context means the scsi_cmnd structure. This completely avoids
dynamic memory allocations for the fast path through I/O submission.
Due the preallocated requests the MQ code path exclusively uses the
host-wide shared tag allocator instead of a per-LUN one. This only
affects drivers actually using the block layer provided tag allocator
instead of their own. Unlike the old path blk-mq always provides a tag,
although drivers don't have to use it.
For now the blk-mq path is disable by defauly and must be enabled using
the "use_blk_mq" module parameter. Once the remaining work in the block
layer to make blk-mq more suitable for slow devices is complete I hope
to make it the default and eventually even remove the old code path.
Based on the earlier scsi-mq prototype by Nicholas Bellinger.
Thanks to Bart Van Assche and Robert Elliot for testing, benchmarking and
various sugestions and code contributions.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Webb Scales <webbnh@hp.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Robert Elliott <elliott@hp.com>
diff --git a/include/scsi/scsi_tcq.h b/include/scsi/scsi_tcq.h
index 81dd12e..cdcc90b 100644
--- a/include/scsi/scsi_tcq.h
+++ b/include/scsi/scsi_tcq.h
@@ -67,7 +67,8 @@
if (!sdev->tagged_supported)
return;
- if (!blk_queue_tagged(sdev->request_queue))
+ if (!shost_use_blk_mq(sdev->host) &&
+ blk_queue_tagged(sdev->request_queue))
blk_queue_init_tags(sdev->request_queue, depth,
sdev->host->bqt);
@@ -80,7 +81,8 @@
**/
static inline void scsi_deactivate_tcq(struct scsi_device *sdev, int depth)
{
- if (blk_queue_tagged(sdev->request_queue))
+ if (!shost_use_blk_mq(sdev->host) &&
+ blk_queue_tagged(sdev->request_queue))
blk_queue_free_tags(sdev->request_queue);
scsi_adjust_queue_depth(sdev, 0, depth);
}
@@ -108,6 +110,15 @@
return 0;
}
+static inline struct scsi_cmnd *scsi_mq_find_tag(struct Scsi_Host *shost,
+ unsigned int hw_ctx, int tag)
+{
+ struct request *req;
+
+ req = blk_mq_tag_to_rq(shost->tag_set.tags[hw_ctx], tag);
+ return req ? (struct scsi_cmnd *)req->special : NULL;
+}
+
/**
* scsi_find_tag - find a tagged command by device
* @SDpnt: pointer to the ScSI device
@@ -118,10 +129,12 @@
**/
static inline struct scsi_cmnd *scsi_find_tag(struct scsi_device *sdev, int tag)
{
-
struct request *req;
if (tag != SCSI_NO_TAG) {
+ if (shost_use_blk_mq(sdev->host))
+ return scsi_mq_find_tag(sdev->host, 0, tag);
+
req = blk_queue_find_tag(sdev->request_queue, tag);
return req ? (struct scsi_cmnd *)req->special : NULL;
}
@@ -130,6 +143,7 @@
return sdev->current_cmnd;
}
+
/**
* scsi_init_shared_tag_map - create a shared tag map
* @shost: the host to share the tag map among all devices
@@ -138,6 +152,12 @@
static inline int scsi_init_shared_tag_map(struct Scsi_Host *shost, int depth)
{
/*
+ * We always have a shared tag map around when using blk-mq.
+ */
+ if (shost_use_blk_mq(shost))
+ return 0;
+
+ /*
* If the shared tag map isn't already initialized, do it now.
* This saves callers from having to check ->bqt when setting up
* devices on the shared host (for libata)
@@ -165,6 +185,8 @@
struct request *req;
if (tag != SCSI_NO_TAG) {
+ if (shost_use_blk_mq(shost))
+ return scsi_mq_find_tag(shost, 0, tag);
req = blk_map_queue_find_tag(shost->bqt, tag);
return req ? (struct scsi_cmnd *)req->special : NULL;
}