[PATCHSET/RFC] Make legacy IO schedulers work with blk-mq

Commit Message

Jens Axboe Dec. 3, 2016, 4:04 a.m. UTC

On 12/02/2016 09:02 PM, Jens Axboe wrote:
> On 12/02/2016 08:15 PM, Jens Axboe wrote:
>> This is by no means done, but it seems to work well enough that
>> I thought I'd send it out for others to take a look at and play
>> with.
>>
>> Basically this allows blk-mq managed devices to run the legacy
>> IO schedulers, unmodified. The only requirement is that the
>> blk-mq device has to be single queue for now, though that
>> limitation would be rather simple to lift.
>>
>> Since this is a debug patch, the default scheduler is deadline.
>> You can switch that to the other configured schedulers, as you
>> would with non-mq devices. Here's an example of a scsi-mq device
>> that is running deadline, and being switched to CFQ online:
>>
>> root@leopard:~# cat /sys/block/sda/mq/0/tags 
>> nr_tags=31, reserved_tags=0, bits_per_word=4
>> nr_free=31, nr_reserved=0
>> active_queues=0
>>
>> root@leopard:~# cat /sys/block/sda/queue/scheduler 
>> noop [deadline] cfq 
>>
>> root@leopard:~# echo cfq > /sys/block/sda/queue/scheduler 
>> root@leopard:~# cat /sys/block/sda/queue/scheduler 
>> noop deadline [cfq] 
>>
>> Testing welcome. There's certainly room for improvement here, so
>> I'm mostly interested in grave performance issues or crashes, if any.
>>
>> Can also be viewed/fetched via git:
>>
>> git://git.kernel.dk/linux-block for-4.11/blk-mq-legacy-sched
> 
> BTW, didn't include the patch for SCSI. You need the below to enable
> scheduling on SCSI devices.

Gah, that was reversed, time to put the computer away for the night.
Below is correct.

commit 8eea81e0903fcde1c28044ea66acc4c5c578f553
Author: Jens Axboe <[e-mail]>
Date:   Fri Dec 2 21:03:43 2016 -0700

    scsi: enable IO scheduling for scsi-mq
    
    Signed-off-by: Jens Axboe <[e-mail]>

Patch

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 47a5c8783b89..aedcec30ee5f 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2121,7 +2121,7 @@  int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	shost->tag_set.queue_depth = shost->can_queue;
 	shost->tag_set.cmd_size = cmd_size;
 	shost->tag_set.numa_node = NUMA_NO_NODE;
-	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
+	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE | BLK_MQ_F_SQ_SCHED;
 	shost->tag_set.flags |=
 		BLK_ALLOC_POLICY_TO_MQ_FLAG(shost->hostt->tag_alloc_policy);
 	shost->tag_set.driver_data = shost;