[PATCH v2] blk-ioprio: Introduce promote-to-rt policy

[Hou Tao]

From: Hou Tao <houtao1@xxxxxxxxxx>

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 ioprio policy to promote the iopriority of bio to
IOPRIO_CLASS_RT if the ioprio is not already RT.

So introduce a new promote-to-rt policy to achieve this. 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 allow for a smoother
transition, just keep it and treat it as an alias of the promote-to-rt
policy.

Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
---
v2:
 * Simplify the implementation of promote-to-rt (from Bart)
 * Make none-to-rt to work again by treating it as an alias of
   the promote-to-rt policy (from Bart & Jan)
 * fix the style of new content in cgroup-v2.rst (from Bagas)
 * set the default priority level to 4 instead of 0 for promote-to-rt

v1: https://lore.kernel.org/linux-block/20230201045227.2203123-1-houtao@xxxxxxxxxxxxxxx

 Documentation/admin-guide/cgroup-v2.rst | 42 ++++++++++++++-----------
 block/blk-ioprio.c                      | 23 ++++++++++++--
 2 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 74cec76be9f2..ccfb9fdfbc16 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2021,31 +2021,33 @@ that attribute:
   no-change
 	Do not modify the I/O priority class.
 
-  none-to-rt
-	For requests that do not have an I/O priority class (NONE),
-	change the I/O priority class into RT. Do not modify
-	the I/O priority class of other requests.
+  promote-to-rt
+	For requests that have a no-RT I/O priority class, change it into RT.
+	Also change the priority level of these requests to 4. Do not modify
+	the I/O priority of requests that have priority class RT.
 
   restrict-to-be
 	For requests that do not have an I/O priority class or that have I/O
-	priority class RT, change it into BE. Do not modify the I/O priority
-	class of requests that have priority class IDLE.
+	priority class RT, change it into BE. Also change the priority level
+	of these requests to 0. Do not modify the I/O priority class of
+	requests that have priority class IDLE.
 
   idle
 	Change the I/O priority class of all requests into IDLE, the lowest
 	I/O priority class.
 
+  none-to-rt
+	Deprecated. Just an alias for promote-to-rt.
+
 The following numerical values are associated with the I/O priority policies:
 
-+-------------+---+
-| no-change   | 0 |
-+-------------+---+
-| none-to-rt  | 1 |
-+-------------+---+
-| rt-to-be    | 2 |
-+-------------+---+
-| all-to-idle | 3 |
-+-------------+---+
++----------------+---+
+| no-change      | 0 |
++----------------+---+
+| rt-to-be       | 2 |
++----------------+---+
+| all-to-idle    | 3 |
++----------------+---+
 
 The numerical value that corresponds to each I/O priority class is as follows:
 
@@ -2061,9 +2063,13 @@ The numerical value that corresponds to each I/O priority class is as follows:
 
 The algorithm to set the I/O priority class for a request is as follows:
 
-- Translate the I/O priority class policy into a number.
-- Change the request I/O priority class into the maximum of the I/O priority
-  class policy number and the numerical I/O priority class.
+- If I/O priority class policy is promote-to-rt, change the request I/O
+  priority class to IOPRIO_CLASS_RT and change the request I/O priority
+  level to 4.
+- If I/O priorityt class is not promote-to-rt, translate the I/O priority
+  class policy into a number, then change the request I/O priority class
+  into the maximum of the I/O priority class policy number and the numerical
+  I/O priority class.
 
 PID
 ---
diff --git a/block/blk-ioprio.c b/block/blk-ioprio.c
index 8bb6b8eba4ce..4eba569d4823 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
-- 
2.29.2