From d285203cf647d7c97db3a1c33794315c9008593f Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Fri, 17 Jan 2014 12:06:53 +0100 Subject: 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 Reviewed-by: Martin K. Petersen Reviewed-by: Hannes Reinecke Reviewed-by: Webb Scales Acked-by: Jens Axboe Tested-by: Bart Van Assche Tested-by: Robert Elliott --- drivers/scsi/scsi_sysfs.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'drivers/scsi/scsi_sysfs.c') diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c index 209cae3097ea..406b3038bbad 100644 --- a/drivers/scsi/scsi_sysfs.c +++ b/drivers/scsi/scsi_sysfs.c @@ -333,6 +333,7 @@ store_shost_eh_deadline(struct device *dev, struct device_attribute *attr, static DEVICE_ATTR(eh_deadline, S_IRUGO | S_IWUSR, show_shost_eh_deadline, store_shost_eh_deadline); +shost_rd_attr(use_blk_mq, "%d\n"); shost_rd_attr(unique_id, "%u\n"); shost_rd_attr(cmd_per_lun, "%hd\n"); shost_rd_attr(can_queue, "%hd\n"); @@ -352,6 +353,7 @@ show_host_busy(struct device *dev, struct device_attribute *attr, char *buf) static DEVICE_ATTR(host_busy, S_IRUGO, show_host_busy, NULL); static struct attribute *scsi_sysfs_shost_attrs[] = { + &dev_attr_use_blk_mq.attr, &dev_attr_unique_id.attr, &dev_attr_host_busy.attr, &dev_attr_cmd_per_lun.attr, -- cgit v1.2.3