block: Introduce the ioprio rq-qos policy

Introduce an rq-qos policy that assigns an I/O priority to requests based
on blk-cgroup configuration settings. This policy has the following
advantages over the ioprio_set() system call:
- This policy is cgroup based so it has all the advantages of cgroups.
- While ioprio_set() does not affect page cache writeback I/O, this rq-qos
  controller affects page cache writeback I/O for filesystems that support
  assiociating a cgroup with writeback I/O. See also
  Documentation/admin-guide/cgroup-v2.rst.

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210618004456.7280-5-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 55 insertions, 0 deletions

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index b1e81aa8598a..4e59925e6583 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -56,6 +56,7 @@ v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgrou
        5-3-3. IO Latency
          5-3-3-1. How IO Latency Throttling Works
          5-3-3-2. IO Latency Interface Files
+       5-3-4. IO Priority
      5-4. PID
        5-4-1. PID Interface Files
      5-5. Cpuset
@@ -1866,6 +1867,60 @@ IO Latency Interface Files
 		duration of time between evaluation events.  Windows only elapse
 		with IO activity.  Idle periods extend the most recent window.
 
+IO Priority
+~~~~~~~~~~~
+
+A single attribute controls the behavior of the I/O priority cgroup policy,
+namely the blkio.prio.class attribute. The following values are accepted for
+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.
+
+  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.
+
+  idle
+	Change the I/O priority class of all requests into IDLE, the lowest
+	I/O priority class.
+
+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 |
++-------------+---+
+
+The numerical value that corresponds to each I/O priority class is as follows:
+
++-------------------------------+---+
+| IOPRIO_CLASS_NONE             | 0 |
++-------------------------------+---+
+| IOPRIO_CLASS_RT (real-time)   | 1 |
++-------------------------------+---+
+| IOPRIO_CLASS_BE (best effort) | 2 |
++-------------------------------+---+
+| IOPRIO_CLASS_IDLE             | 3 |
++-------------------------------+---+
+
+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.
+
 PID
 ---