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>

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index f67c082..7544ce0 100644 (file)
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2024,31 +2024,33 @@ 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 non-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:
 
@@ -2064,9 +2066,13 @@ 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 055529b..4051fad 100644 (file)
--- 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