Re: [PATCH scsi-misc-2.6 01/05] scsi: make blk layer set REQ_SOFTBARRIER when a request is dispatched

Tejun Heo
2005-04-20T06:44:58Z

Jens Axboe wrote:
> On Wed, Apr 20 2005, Tejun Heo wrote:
>
>>01_scsi_blk_make_started_requests_ordered.patch
>>
>> Reordering already started requests is without any real
>> benefit and causes problems if the request has its
>> driver-specific resources allocated (as in SCSI). This patch
>> makes elv_next_request() set REQ_SOFTBARRIER automatically
>> when a request is dispatched.
>>
>> As both as and cfq schedulers don't allow passing requeued
>> requests, the only behavior change is that requests deferred
>> by prep_fn won't be passed by other requests. This change
>> shouldn't cause any problem. The only affected driver other
>> than SCSI is i2o_block.
>>
>>Signed-off-by: Tejun Heo <htejun@gmail.com>
>>
>> elevator.c | 8 ++++----
>> 1 files changed, 4 insertions(+), 4 deletions(-)
>>
>>Index: scsi-reqfn-export/drivers/block/elevator.c
>>===================================================================
>>--- scsi-reqfn-export.orig/drivers/block/elevator.c 2005-04-20 08:13:01.000000000 +0900
>>+++ scsi-reqfn-export/drivers/block/elevator.c 2005-04-20 08:13:33.000000000 +0900
>>@@ -370,11 +370,11 @@ struct request *elv_next_request(request
>>
>> while ((rq = __elv_next_request(q)) != NULL) {
>> /*
>>- * just mark as started even if we don't start it, a request
>>- * that has been delayed should not be passed by new incoming
>>- * requests
>>+ * just mark as started even if we don't start it.
>>+ * also, as a request that has been delayed should not
>>+ * be passed by new incoming requests, set softbarrier.
>> */
>>- rq->flags |= REQ_STARTED;
>>+ rq->flags |= REQ_STARTED | REQ_SOFTBARRIER;
>>
>> if (rq == q->last_merge)
>> q->last_merge = NULL;
>
>
> Do it on requeue, please - not on the initial spotting of the request.
>

The thing is that we also need to set REQ_SOFTBARRIER on
BLKPREP_DEFER. So, it will be two places - in elv_next_request and
blk_requeue_request. The end result will be the same. Do you think
doing on requeue paths is better?

--
tejun