[PATCH 3/3] scsi: core: Improve IOPS in case of host-wide tags

Bart Van Assche
2025-09-10T21:33:38Z

The SCSI core uses the budget map to enforce the cmd_per_lun limit.
That limit cannot be exceeded if host->cmd_per_lun >= host->can_queue
and if the host tag set is shared across all hardware queues.
Since scsi_mq_get_budget() shows up in all CPU profiles for fast SCSI
devices, do not allocate a budget map if cmd_per_lun >= can_queue and
if the host tag set is shared across all hardware queues.

On my UFS 4 test setup this patch improves IOPS by 1% and reduces the
time spent in scsi_mq_get_budget() from 0.22% to 0.01%.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
drivers/scsi/scsi.c | 7 ++++-
drivers/scsi/scsi_lib.c | 60 +++++++++++++++++++++++++++++++++-----
drivers/scsi/scsi_scan.c | 11 ++++++-
include/scsi/scsi_device.h | 5 +---
4 files changed, 70 insertions(+), 13 deletions(-)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 9a0f467264b3..06066b694d8a 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -216,6 +216,8 @@ int scsi_device_max_queue_depth(struct scsi_device *sdev)
*/
int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
{
+ struct Scsi_Host *shost = sdev->host;
+
depth = min_t(int, depth, scsi_device_max_queue_depth(sdev));

if (depth > 0) {
@@ -226,7 +228,10 @@ int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
if (sdev->request_queue)
blk_set_queue_depth(sdev->request_queue, depth);

- sbitmap_resize(&sdev->budget_map, sdev->queue_depth);
+ if (shost->host_tagset && depth >= shost->can_queue)
+ sbitmap_free(&sdev->budget_map);
+ else
+ sbitmap_resize(&sdev->budget_map, sdev->queue_depth);

return sdev->queue_depth;
}
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0c65ecfedfbd..c546514d1049 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -396,7 +396,8 @@ void scsi_device_unbusy(struct scsi_device *sdev, struct scsi_cmnd *cmd)
if (starget->can_queue > 0)
atomic_dec(&starget->target_busy);

- sbitmap_put(&sdev->budget_map, cmd->budget_token);
+ if (sdev->budget_map.map)
+ sbitmap_put(&sdev->budget_map, cmd->budget_token);
cmd->budget_token = -1;
}

@@ -445,6 +446,47 @@ static void scsi_single_lun_run(struct scsi_device *current_sdev)
spin_unlock_irqrestore(shost->host_lock, flags);
}

+struct sdev_in_flight_data {
+ const struct scsi_device *sdev;
+ int count;
+};
+
+static bool scsi_device_check_in_flight(struct request *rq, void *data)
+{
+ struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
+ struct sdev_in_flight_data *sifd = data;
+
+ if (cmd->device == sifd->sdev)
+ sifd->count++;
+
+ return true;
+}
+
+/**
+ * scsi_device_busy() - Number of commands allocated for a SCSI device
+ * @sdev: SCSI device.
+ *
+ * Note: There is a subtle difference between this function and
+ * scsi_host_busy(). scsi_host_busy() counts the number of commands that have
+ * been started. This function counts the number of commands that have been
+ * allocated. At least the UFS driver depends on this function counting commands
+ * that have already been allocated but that have not yet been started.
+ */
+int scsi_device_busy(const struct scsi_device *sdev)
+{
+ struct sdev_in_flight_data sifd = { .sdev = sdev };
+ struct blk_mq_tag_set *set = &sdev->host->tag_set;
+
+ if (sdev->budget_map.map)
+ return sbitmap_weight(&sdev->budget_map);
+ if (WARN_ON_ONCE(!set->shared_tags))
+ return 0;
+ blk_mq_all_tag_iter(set->shared_tags, scsi_device_check_in_flight,
+ &sifd);
+ return sifd.count;
+}
+EXPORT_SYMBOL(scsi_device_busy);
+
static inline bool scsi_device_is_busy(struct scsi_device *sdev)
{
if (scsi_device_busy(sdev) >= sdev->queue_depth)
@@ -1358,11 +1400,13 @@ scsi_device_state_check(struct scsi_device *sdev, struct request *req)
static inline int scsi_dev_queue_ready(struct request_queue *q,
struct scsi_device *sdev)
{
- int token;
+ int token = INT_MAX;

- token = sbitmap_get(&sdev->budget_map);
- if (token < 0)
- return -1;
+ if (sdev->budget_map.map) {
+ token = sbitmap_get(&sdev->budget_map);
+ if (token < 0)
+ return -1;
+ }

if (!atomic_read(&sdev->device_blocked))
return token;
@@ -1373,7 +1417,8 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
*/
if (scsi_device_busy(sdev) > 1 ||
atomic_dec_return(&sdev->device_blocked) > 0) {
- sbitmap_put(&sdev->budget_map, token);
+ if (sdev->budget_map.map)
+ sbitmap_put(&sdev->budget_map, token);
return -1;
}

@@ -1749,7 +1794,8 @@ static void scsi_mq_put_budget(struct request_queue *q, int budget_token)
{
struct scsi_device *sdev = q->queuedata;

- sbitmap_put(&sdev->budget_map, budget_token);
+ if (sdev->budget_map.map)
+ sbitmap_put(&sdev->budget_map, budget_token);
}

/*
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 3c6e089e80c3..6f2d0bf0e3ec 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -218,6 +218,7 @@ static void scsi_unlock_floptical(struct scsi_device *sdev,
static int scsi_realloc_sdev_budget_map(struct scsi_device *sdev,
unsigned int depth)
{
+ struct Scsi_Host *shost = sdev->host;
int new_shift = sbitmap_calculate_shift(depth);
bool need_alloc = !sdev->budget_map.map;
bool need_free = false;
@@ -225,6 +226,13 @@ static int scsi_realloc_sdev_budget_map(struct scsi_device *sdev,
int ret;
struct sbitmap sb_backup;

+ if (shost->host_tagset && depth >= shost->can_queue) {
+ memflags = blk_mq_freeze_queue(sdev->request_queue);
+ sbitmap_free(&sb_backup);
+ blk_mq_unfreeze_queue(sdev->request_queue, memflags);
+ return 0;
+ }
+
depth = min_t(unsigned int, depth, scsi_device_max_queue_depth(sdev));

/*
@@ -1112,7 +1120,8 @@ static int scsi_add_lun(struct scsi_device *sdev, unsigned char *inq_result,
scsi_cdl_check(sdev);

sdev->max_queue_depth = sdev->queue_depth;
- WARN_ON_ONCE(sdev->max_queue_depth > sdev->budget_map.depth);
+ WARN_ON_ONCE(sdev->budget_map.map &&
+ sdev->max_queue_depth > sdev->budget_map.depth);
sdev->sdev_bflags = *bflags;

/*
diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index 6d6500148c4b..3c7a95fa9b67 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -687,10 +687,7 @@ static inline int scsi_device_supports_vpd(struct scsi_device *sdev)
return 0;
}

-static inline int scsi_device_busy(struct scsi_device *sdev)
-{
- return sbitmap_weight(&sdev->budget_map);
-}
+int scsi_device_busy(const struct scsi_device *sdev);

/* Macros to access the UNIT ATTENTION counters */
#define scsi_get_ua_new_media_ctr(sdev) \