blk-ioprio: Introduce promote-to-rt policy

Since commit a78418e6a04c ("block: Always initialize bio IO priority on
submit"), bio->bi_ioprio will never be IOPRIO_CLASS_NONE when calling
blkcg_set_ioprio(), so there will be no way to promote the io-priority
of one cgroup to IOPRIO_CLASS_RT, because bi_ioprio will always be
greater than or equals to IOPRIO_CLASS_RT.

It seems possible to call blkcg_set_ioprio() first then try to
initialize bi_ioprio later in bio_set_ioprio(), but this doesn't work
for bio in which bi_ioprio is already initialized (e.g., direct-io), so
introduce a new promote-to-rt policy to promote the iopriority of bio to
IOPRIO_CLASS_RT if the ioprio is not already RT.

For none-to-rt policy, although it doesn't work now, but considering
that its purpose was also to override the io-priority to RT and allowing
for a smoother transition, just keep it and treat it as an alias of
the promote-to-rt policy.

Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20230428074404.280532-1-houtao@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 20 insertions, 3 deletions

diff --git a/block/blk-ioprio.c b/block/blk-ioprio.c
index 055529b9b92b..4051fada01f1 100644
--- a/block/blk-ioprio.c
+++ b/block/blk-ioprio.c
@@ -23,25 +23,28 @@
 /**
  * enum prio_policy - I/O priority class policy.
  * @POLICY_NO_CHANGE: (default) do not modify the I/O priority class.
- * @POLICY_NONE_TO_RT: modify IOPRIO_CLASS_NONE into IOPRIO_CLASS_RT.
+ * @POLICY_PROMOTE_TO_RT: modify no-IOPRIO_CLASS_RT to IOPRIO_CLASS_RT.
  * @POLICY_RESTRICT_TO_BE: modify IOPRIO_CLASS_NONE and IOPRIO_CLASS_RT into
  *		IOPRIO_CLASS_BE.
  * @POLICY_ALL_TO_IDLE: change the I/O priority class into IOPRIO_CLASS_IDLE.
+ * @POLICY_NONE_TO_RT: an alias for POLICY_PROMOTE_TO_RT.
  *
  * See also <linux/ioprio.h>.
  */
 enum prio_policy {
 	POLICY_NO_CHANGE	= 0,
-	POLICY_NONE_TO_RT	= 1,
+	POLICY_PROMOTE_TO_RT	= 1,
 	POLICY_RESTRICT_TO_BE	= 2,
 	POLICY_ALL_TO_IDLE	= 3,
+	POLICY_NONE_TO_RT	= 4,
 };
 
 static const char *policy_name[] = {
 	[POLICY_NO_CHANGE]	= "no-change",
-	[POLICY_NONE_TO_RT]	= "none-to-rt",
+	[POLICY_PROMOTE_TO_RT]	= "promote-to-rt",
 	[POLICY_RESTRICT_TO_BE]	= "restrict-to-be",
 	[POLICY_ALL_TO_IDLE]	= "idle",
+	[POLICY_NONE_TO_RT]	= "none-to-rt",
 };
 
 static struct blkcg_policy ioprio_policy;
@@ -189,6 +192,20 @@ void blkcg_set_ioprio(struct bio *bio)
 	if (!blkcg || blkcg->prio_policy == POLICY_NO_CHANGE)
 		return;
 
+	if (blkcg->prio_policy == POLICY_PROMOTE_TO_RT ||
+	    blkcg->prio_policy == POLICY_NONE_TO_RT) {
+		/*
+		 * For RT threads, the default priority level is 4 because
+		 * task_nice is 0. By promoting non-RT io-priority to RT-class
+		 * and default level 4, those requests that are already
+		 * RT-class but need a higher io-priority can use ioprio_set()
+		 * to achieve this.
+		 */
+		if (IOPRIO_PRIO_CLASS(bio->bi_ioprio) != IOPRIO_CLASS_RT)
+			bio->bi_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_RT, 4);
+		return;
+	}
+
 	/*
 	 * Except for IOPRIO_CLASS_NONE, higher I/O priority numbers
 	 * correspond to a lower priority. Hence, the max_t() below selects