[PATCH] SCSI: don't get target/host busy_count in scsi_mq_get_budget()

Ming Lei
2017-11-04T01:55:57Z

It is very expensive to atomic_inc/atomic_dec the host wide counter of
host->busy_count, and it should have been avoided via blk-mq's mechanism
of getting driver tag, which uses the more efficient way of sbitmap queue.

Also we don't check atomic_read(&sdev->device_busy) in scsi_mq_get_budget()
and don't run queue if the counter becomes zero, so IO hang may be caused
if all requests are completed just before the current SCSI device
is added to shost->starved_list.

Fixes: 0df21c86bdbf(scsi: implement .get_budget and .put_budget for blk-mq)
Reported-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---

drivers/scsi/scsi_lib.c | 29 +++++++++++++----------------
1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index a098af3070a1..7f218ef61900 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1944,11 +1944,7 @@ static void scsi_mq_put_budget(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct scsi_device *sdev = q->queuedata;
- struct Scsi_Host *shost = sdev->host;

- atomic_dec(&shost->host_busy);
- if (scsi_target(sdev)->can_queue > 0)
- atomic_dec(&scsi_target(sdev)->target_busy);
atomic_dec(&sdev->device_busy);
put_device(&sdev->sdev_gendev);
}
@@ -1957,7 +1953,6 @@ static blk_status_t scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct scsi_device *sdev = q->queuedata;
- struct Scsi_Host *shost = sdev->host;
blk_status_t ret;

ret = prep_to_mq(scsi_prep_state_check(sdev, NULL));
@@ -1968,18 +1963,9 @@ static blk_status_t scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
goto out;
if (!scsi_dev_queue_ready(q, sdev))
goto out_put_device;
- if (!scsi_target_queue_ready(shost, sdev))
- goto out_dec_device_busy;
- if (!scsi_host_queue_ready(q, shost, sdev))
- goto out_dec_target_busy;

return BLK_STS_OK;

-out_dec_target_busy:
- if (scsi_target(sdev)->can_queue > 0)
- atomic_dec(&scsi_target(sdev)->target_busy);
-out_dec_device_busy:
- atomic_dec(&sdev->device_busy);
out_put_device:
put_device(&sdev->sdev_gendev);
out:
@@ -1992,6 +1978,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
struct request *req = bd->rq;
struct request_queue *q = req->q;
struct scsi_device *sdev = q->queuedata;
+ struct Scsi_Host *shost = sdev->host;
struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
blk_status_t ret;
int reason;
@@ -2001,10 +1988,15 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
goto out_put_budget;

ret = BLK_STS_RESOURCE;
+ if (!scsi_target_queue_ready(shost, sdev))
+ goto out_put_budget;
+ if (!scsi_host_queue_ready(q, shost, sdev))
+ goto out_dec_target_busy;
+
if (!(req->rq_flags & RQF_DONTPREP)) {
ret = prep_to_mq(scsi_mq_prep_fn(req));
if (ret != BLK_STS_OK)
- goto out_put_budget;
+ goto out_dec_host_busy;
req->rq_flags |= RQF_DONTPREP;
} else {
blk_mq_start_request(req);
@@ -2022,11 +2014,16 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
if (reason) {
scsi_set_blocked(cmd, reason);
ret = BLK_STS_RESOURCE;
- goto out_put_budget;
+ goto out_dec_host_busy;
}

return BLK_STS_OK;

+out_dec_host_busy:
+ atomic_dec(&shost->host_busy);
+out_dec_target_busy:
+ if (scsi_target(sdev)->can_queue > 0)
+ atomic_dec(&scsi_target(sdev)->target_busy);
out_put_budget:
scsi_mq_put_budget(hctx);
switch (ret) {
--
2.9.5