Commit ddf63516 authored by Hou Tao's avatar Hou Tao Committed by Jens Axboe
Browse files

blk-ioprio: Introduce promote-to-rt policy 



Since commit a78418e6 ("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: default avatarTejun Heo <tj@kernel.org>
Reviewed-by: default avatarBart Van Assche <bvanassche@acm.org>
Reviewed-by: default avatarChaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarHou Tao <houtao1@huawei.com>
Reviewed-by: default avatarBagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20230428074404.280532-1-houtao@huaweicloud.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 8d211554

Documentation/admin-guide/cgroup-v2.rst

+24 −18
Original line number Diff line number Diff line
@@ -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 |

+-------------+---+

+----------------+---+



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

---

block/blk-ioprio.c

+20 −3
Original line number Diff line number Diff line
@@ -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