[RFC PATCH 6/6] scsi: add shared-tag fairness to host_tagset drivers

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

Introduce a per-host shared_pct into the scsi block tag set so
host_tagset drivers can choose how much of the shared pool stays carved
into per-LUN exclusivity versus shared by everyone. This may be useful
for UFS where the sharing is harmful to performance.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
 drivers/scsi/scsi_debug.c | 8 +++++++-
 drivers/scsi/scsi_lib.c   | 4 +++-
 include/scsi/scsi_host.h  | 8 ++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 9d1c9c41d0f99..cb2bec76ba3de 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -905,6 +905,7 @@ static int sdebug_every_nth = DEF_EVERY_NTH;
 static int sdebug_fake_rw = DEF_FAKE_RW;
 static unsigned int sdebug_guard = DEF_GUARD;
 static int sdebug_host_max_queue;	/* per host */
+static int sdebug_shared_pct;		/* host_tagset shared tag pool % */
 static int sdebug_lowest_aligned = DEF_LOWEST_ALIGNED;
 static int sdebug_max_luns = DEF_MAX_LUNS;
 static int sdebug_max_queue = SDEBUG_CANQUEUE;	/* per submit queue */
@@ -7343,6 +7344,7 @@ module_param_named(fake_rw, sdebug_fake_rw, int, S_IRUGO | S_IWUSR);
 module_param_named(guard, sdebug_guard, uint, S_IRUGO);
 module_param_named(host_lock, sdebug_host_lock, bool, S_IRUGO | S_IWUSR);
 module_param_named(host_max_queue, sdebug_host_max_queue, int, S_IRUGO);
+module_param_named(shared_pct, sdebug_shared_pct, int, S_IRUGO);
 module_param_string(inq_product, sdebug_inq_product_id,
 		    sizeof(sdebug_inq_product_id), S_IRUGO | S_IWUSR);
 module_param_string(inq_rev, sdebug_inq_product_rev,
@@ -7427,6 +7429,8 @@ MODULE_PARM_DESC(guard, "protection checksum: 0=crc, 1=ip (def=0)");
 MODULE_PARM_DESC(host_lock, "host_lock is ignored (def=0)");
 MODULE_PARM_DESC(host_max_queue,
 		 "host max # of queued cmds (0 to max(def) [max_queue fixed equal for !0])");
+MODULE_PARM_DESC(shared_pct,
+		 "host_tagset: %% of the shared tag pool shared by all LUNs (0=exclusive, 100=no fairness, def=0)");
 MODULE_PARM_DESC(inq_product, "SCSI INQUIRY product string (def=\"scsi_debug\")");
 MODULE_PARM_DESC(inq_rev, "SCSI INQUIRY revision string (def=\""
 		 SDEBUG_VERSION "\")");
@@ -9569,8 +9573,10 @@ static int sdebug_driver_probe(struct device *dev)
 	 * following should give the same answer for each host.
 	 */
 	hpnt->nr_hw_queues = submit_queues;
-	if (sdebug_host_max_queue)
+	if (sdebug_host_max_queue) {
 		hpnt->host_tagset = 1;
+		hpnt->shared_pct = sdebug_shared_pct;
+	}
 
 	/* poll queues are possible for nr_hw_queues > 1 */
 	if (hpnt->nr_hw_queues == 1 || (poll_queues < 1)) {
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 22e2e3223440d..efcb49af899a2 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2152,8 +2152,10 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	if (shost->queuecommand_may_block)
 		tag_set->flags |= BLK_MQ_F_BLOCKING;
 	tag_set->driver_data = shost;
-	if (shost->host_tagset)
+	if (shost->host_tagset) {
 		tag_set->flags |= BLK_MQ_F_TAG_HCTX_SHARED;
+		tag_set->shared_pct = shost->shared_pct;
+	}
 
 	return blk_mq_alloc_tag_set(tag_set);
 }
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 7e2011830ba4b..68c99bc7ad865 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -694,6 +694,14 @@ struct Scsi_Host {
 	/* The queuecommand callback may block. See also BLK_MQ_F_BLOCKING. */
 	unsigned queuecommand_may_block:1;
 
+	/*
+	 * For host_tagset hosts: percent of the shared tag pool made available
+	 * to all LUNs rather than reserved as per-LUN exclusive floors. See
+	 * blk_mq_tag_set.shared_pct. 0 (default) divides the pool exclusively;
+	 * 100 lets every LUN allocate from the whole pool with no fairness.
+	 */
+	unsigned int shared_pct;
+
 	/* Host responded with short (<36 bytes) INQUIRY result */
 	unsigned short_inquiry:1;
 
-- 
2.52.0