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>

1 files changed, 30 insertions, 5 deletions

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 0632eee82620..6de80e352871 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -213,9 +213,24 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
                 goto fail;
         }
 
+        if (shost_use_blk_mq(shost)) {
+                error = scsi_mq_setup_tags(shost);
+                if (error)
+                        goto fail;
+        }
+
+        /*
+         * Note that we allocate the freelist even for the MQ case for now,
+         * as we need a command set aside for scsi_reset_provider.  Having
+         * the full host freelist and one command available for that is a
+         * little heavy-handed, but avoids introducing a special allocator
+         * just for this.  Eventually the structure of scsi_reset_provider
+         * will need a major overhaul.
+         */
         error = scsi_setup_command_freelist(shost);
         if (error)
-                goto fail;
+                goto out_destroy_tags;
+
 
         if (!shost->shost_gendev.parent)
                 shost->shost_gendev.parent = dev ? dev : &platform_bus;
@@ -226,7 +241,7 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 
         error = device_add(&shost->shost_gendev);
         if (error)
-                goto out;
+                goto out_destroy_freelist;
 
         pm_runtime_set_active(&shost->shost_gendev);
         pm_runtime_enable(&shost->shost_gendev);
@@ -279,8 +294,11 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
         device_del(&shost->shost_dev);
  out_del_gendev:
         device_del(&shost->shost_gendev);
- out:
+ out_destroy_freelist:
         scsi_destroy_command_freelist(shost);
+ out_destroy_tags:
+        if (shost_use_blk_mq(shost))
+                scsi_mq_destroy_tags(shost);
  fail:
         return error;
 }
@@ -309,8 +327,13 @@ static void scsi_host_dev_release(struct device *dev)
         }
 
         scsi_destroy_command_freelist(shost);
-        if (shost->bqt)
-                blk_free_tags(shost->bqt);
+        if (shost_use_blk_mq(shost)) {
+                if (shost->tag_set.tags)
+                        scsi_mq_destroy_tags(shost);
+        } else {
+                if (shost->bqt)
+                        blk_free_tags(shost->bqt);
+        }
 
         kfree(shost->shost_data);
 
@@ -436,6 +459,8 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
         else
                 shost->dma_boundary = 0xffffffff;
 
+        shost->use_blk_mq = scsi_use_blk_mq && !shost->hostt->disable_blk_mq;
+
         device_initialize(&shost->shost_gendev);
         dev_set_name(&shost->shost_gendev, "host%d", shost->host_no);
         shost->shost_gendev.bus = &scsi_bus_type;