Re: [PATCH v3 04/16] block: Introduce the ioprio rq-qos policy

[Adam Manzanares <a.manzanares@samsung.com>]

On Thu, Jun 17, 2021 at 05:44:44PM -0700, Bart Van Assche wrote:
> 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>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  55 +++++
>  block/Kconfig                           |   9 +
>  block/Makefile                          |   1 +
>  block/blk-cgroup.c                      |   5 +
>  block/blk-ioprio.c                      | 262 ++++++++++++++++++++++++
>  block/blk-ioprio.h                      |  19 ++
>  block/blk-mq-debugfs.c                  |   2 +
>  block/blk-rq-qos.h                      |   1 +
>  8 files changed, 354 insertions(+)
>  create mode 100644 block/blk-ioprio.c
>  create mode 100644 block/blk-ioprio.h
> 
> 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
>  ---
>  
> diff --git a/block/Kconfig b/block/Kconfig
> index 6685578b2a20..e71c63eaaf52 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -162,6 +162,15 @@ config BLK_CGROUP_IOCOST
>  	distributes IO capacity between different groups based on
>  	their share of the overall weight distribution.
>  
> +config BLK_CGROUP_IOPRIO
> +	bool "Cgroup I/O controller for assigning an I/O priority class"
> +	depends on BLK_CGROUP
> +	help
> +	Enable the .prio interface for assigning an I/O priority class to
> +	requests. The I/O priority class affects the order in which an I/O
> +	scheduler and block devices process requests. Only some I/O schedulers
> +	and some block devices support I/O priorities.
> +
>  config BLK_DEBUG_FS
>  	bool "Block layer debugging information in debugfs"
>  	default y
> diff --git a/block/Makefile b/block/Makefile
> index 8d841f5f986f..af3d044abaf1 100644
> --- a/block/Makefile
> +++ b/block/Makefile
> @@ -17,6 +17,7 @@ obj-$(CONFIG_BLK_DEV_BSGLIB)	+= bsg-lib.o
>  obj-$(CONFIG_BLK_CGROUP)	+= blk-cgroup.o
>  obj-$(CONFIG_BLK_CGROUP_RWSTAT)	+= blk-cgroup-rwstat.o
>  obj-$(CONFIG_BLK_DEV_THROTTLING)	+= blk-throttle.o
> +obj-$(CONFIG_BLK_CGROUP_IOPRIO)	+= blk-ioprio.o
>  obj-$(CONFIG_BLK_CGROUP_IOLATENCY)	+= blk-iolatency.o
>  obj-$(CONFIG_BLK_CGROUP_IOCOST)	+= blk-iocost.o
>  obj-$(CONFIG_MQ_IOSCHED_DEADLINE)	+= mq-deadline.o
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 3b0f6efaa2b6..7b06a5fa3cac 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -31,6 +31,7 @@
>  #include <linux/tracehook.h>
>  #include <linux/psi.h>
>  #include "blk.h"
> +#include "blk-ioprio.h"
>  
>  /*
>   * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
> @@ -1187,6 +1188,10 @@ int blkcg_init_queue(struct request_queue *q)
>  	if (ret)
>  		goto err_destroy_all;
>  
> +	ret = blk_ioprio_init(q);
> +	if (ret)
> +		goto err_destroy_all;
> +
>  	ret = blk_throtl_init(q);
>  	if (ret)
>  		goto err_destroy_all;
> diff --git a/block/blk-ioprio.c b/block/blk-ioprio.c
> new file mode 100644
> index 000000000000..332a07761bf8
> --- /dev/null
> +++ b/block/blk-ioprio.c
> @@ -0,0 +1,262 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Block rq-qos policy for assigning an I/O priority class to requests.
> + *
> + * Using an rq-qos policy for assigning I/O priority class has two advantages
> + * over using 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.
> + */
> +
> +#include <linux/blk-cgroup.h>
> +#include <linux/blk-mq.h>
> +#include <linux/blk_types.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include "blk-ioprio.h"
> +#include "blk-rq-qos.h"
> +
> +/**
> + * 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_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.
> + *
> + * See also <linux/ioprio.h>.
> + */
> +enum prio_policy {
> +	POLICY_NO_CHANGE	= 0,
> +	POLICY_NONE_TO_RT	= 1,
> +	POLICY_RESTRICT_TO_BE	= 2,
> +	POLICY_ALL_TO_IDLE	= 3,
> +};
> +
> +static const char *policy_name[] = {
> +	[POLICY_NO_CHANGE]	= "no-change",
> +	[POLICY_NONE_TO_RT]	= "none-to-rt",
> +	[POLICY_RESTRICT_TO_BE]	= "restrict-to-be",
> +	[POLICY_ALL_TO_IDLE]	= "idle",
> +};
> +
> +static struct blkcg_policy ioprio_policy;
> +
> +/**
> + * struct ioprio_blkg - Per (cgroup, request queue) data.
> + * @pd: blkg_policy_data structure.
> + */
> +struct ioprio_blkg {
> +	struct blkg_policy_data pd;
> +};
> +
> +/**
> + * struct ioprio_blkcg - Per cgroup data.
> + * @cpd: blkcg_policy_data structure.
> + * @prio_policy: One of the IOPRIO_CLASS_* values. See also <linux/ioprio.h>.
> + */
> +struct ioprio_blkcg {
> +	struct blkcg_policy_data cpd;
> +	enum prio_policy	 prio_policy;
> +};
> +
> +static inline struct ioprio_blkg *pd_to_ioprio(struct blkg_policy_data *pd)
> +{
> +	return pd ? container_of(pd, struct ioprio_blkg, pd) : NULL;
> +}
> +
> +static struct ioprio_blkcg *blkcg_to_ioprio_blkcg(struct blkcg *blkcg)
> +{
> +	return container_of(blkcg_to_cpd(blkcg, &ioprio_policy),
> +			    struct ioprio_blkcg, cpd);
> +}
> +
> +static struct ioprio_blkcg *
> +ioprio_blkcg_from_css(struct cgroup_subsys_state *css)
> +{
> +	return blkcg_to_ioprio_blkcg(css_to_blkcg(css));
> +}
> +
> +static struct ioprio_blkcg *ioprio_blkcg_from_bio(struct bio *bio)
> +{
> +	struct blkg_policy_data *pd = blkg_to_pd(bio->bi_blkg, &ioprio_policy);
> +
> +	if (!pd)
> +		return NULL;
> +
> +	return blkcg_to_ioprio_blkcg(pd->blkg->blkcg);
> +}
> +
> +static int ioprio_show_prio_policy(struct seq_file *sf, void *v)
> +{
> +	struct ioprio_blkcg *blkcg = ioprio_blkcg_from_css(seq_css(sf));
> +
> +	seq_printf(sf, "%s\n", policy_name[blkcg->prio_policy]);
> +	return 0;
> +}
> +
> +static ssize_t ioprio_set_prio_policy(struct kernfs_open_file *of, char *buf,
> +				      size_t nbytes, loff_t off)
> +{
> +	struct ioprio_blkcg *blkcg = ioprio_blkcg_from_css(of_css(of));
> +	int ret;
> +
> +	if (off != 0)
> +		return -EIO;
> +	/* kernfs_fop_write_iter() terminates 'buf' with '\0'. */
> +	ret = sysfs_match_string(policy_name, buf);
> +	if (ret < 0)
> +		return ret;
> +	blkcg->prio_policy = ret;
> +
> +	return nbytes;
> +}
> +
> +static struct blkg_policy_data *
> +ioprio_alloc_pd(gfp_t gfp, struct request_queue *q, struct blkcg *blkcg)
> +{
> +	struct ioprio_blkg *ioprio_blkg;
> +
> +	ioprio_blkg = kzalloc(sizeof(*ioprio_blkg), gfp);
> +	if (!ioprio_blkg)
> +		return NULL;
> +
> +	return &ioprio_blkg->pd;
> +}
> +
> +static void ioprio_free_pd(struct blkg_policy_data *pd)
> +{
> +	struct ioprio_blkg *ioprio_blkg = pd_to_ioprio(pd);
> +
> +	kfree(ioprio_blkg);
> +}
> +
> +static struct blkcg_policy_data *ioprio_alloc_cpd(gfp_t gfp)
> +{
> +	struct ioprio_blkcg *blkcg;
> +
> +	blkcg = kzalloc(sizeof(*blkcg), gfp);
> +	if (!blkcg)
> +		return NULL;
> +	blkcg->prio_policy = POLICY_NO_CHANGE;
> +	return &blkcg->cpd;
> +}
> +
> +static void ioprio_free_cpd(struct blkcg_policy_data *cpd)
> +{
> +	struct ioprio_blkcg *blkcg = container_of(cpd, typeof(*blkcg), cpd);
> +
> +	kfree(blkcg);
> +}
> +
> +#define IOPRIO_ATTRS						\
> +	{							\
> +		.name		= "prio.class",			\
> +		.seq_show	= ioprio_show_prio_policy,	\
> +		.write		= ioprio_set_prio_policy,	\
> +	},							\
> +	{ } /* sentinel */
> +
> +/* cgroup v2 attributes */
> +static struct cftype ioprio_files[] = {
> +	IOPRIO_ATTRS
> +};
> +
> +/* cgroup v1 attributes */
> +static struct cftype ioprio_legacy_files[] = {
> +	IOPRIO_ATTRS
> +};
> +
> +static struct blkcg_policy ioprio_policy = {
> +	.dfl_cftypes	= ioprio_files,
> +	.legacy_cftypes = ioprio_legacy_files,
> +
> +	.cpd_alloc_fn	= ioprio_alloc_cpd,
> +	.cpd_free_fn	= ioprio_free_cpd,
> +
> +	.pd_alloc_fn	= ioprio_alloc_pd,
> +	.pd_free_fn	= ioprio_free_pd,
> +};
> +
> +struct blk_ioprio {
> +	struct rq_qos rqos;
> +};
> +
> +static void blkcg_ioprio_track(struct rq_qos *rqos, struct request *rq,
> +			       struct bio *bio)
> +{
> +	struct ioprio_blkcg *blkcg = ioprio_blkcg_from_bio(bio);
> +
> +	/*
> +	 * Except for IOPRIO_CLASS_NONE, higher I/O priority numbers
> +	 * correspond to a lower priority. Hence, the max_t() below selects
> +	 * the lower priority of bi_ioprio and the cgroup I/O priority class.
> +	 * If the cgroup policy has been set to POLICY_NO_CHANGE == 0, the
> +	 * bio I/O priority is not modified. If the bio I/O priority equals
> +	 * IOPRIO_CLASS_NONE, the cgroup I/O priority is assigned to the bio.
> +	 */
> +	bio->bi_ioprio = max_t(u16, bio->bi_ioprio,
> +			       IOPRIO_PRIO_VALUE(blkcg->prio_policy, 0));
> +}
> +
> +static void blkcg_ioprio_exit(struct rq_qos *rqos)
> +{
> +	struct blk_ioprio *blkioprio_blkg =
> +		container_of(rqos, typeof(*blkioprio_blkg), rqos);
> +
> +	blkcg_deactivate_policy(rqos->q, &ioprio_policy);
> +	kfree(blkioprio_blkg);
> +}
> +
> +static struct rq_qos_ops blkcg_ioprio_ops = {
> +	.track	= blkcg_ioprio_track,
> +	.exit	= blkcg_ioprio_exit,
> +};
> +
> +int blk_ioprio_init(struct request_queue *q)
> +{
> +	struct blk_ioprio *blkioprio_blkg;
> +	struct rq_qos *rqos;
> +	int ret;
> +
> +	blkioprio_blkg = kzalloc(sizeof(*blkioprio_blkg), GFP_KERNEL);
> +	if (!blkioprio_blkg)
> +		return -ENOMEM;
> +
> +	ret = blkcg_activate_policy(q, &ioprio_policy);
> +	if (ret) {
> +		kfree(blkioprio_blkg);
> +		return ret;
> +	}
> +
> +	rqos = &blkioprio_blkg->rqos;
> +	rqos->id = RQ_QOS_IOPRIO;
> +	rqos->ops = &blkcg_ioprio_ops;
> +	rqos->q = q;
> +
> +	/*
> +	 * Registering the rq-qos policy after activating the blk-cgroup
> +	 * policy guarantees that ioprio_blkcg_from_bio(bio) != NULL in the
> +	 * rq-qos callbacks.
> +	 */
> +	rq_qos_add(q, rqos);
> +
> +	return 0;
> +}
> +
> +static int __init ioprio_init(void)
> +{
> +	return blkcg_policy_register(&ioprio_policy);
> +}
> +
> +static void __exit ioprio_exit(void)
> +{
> +	blkcg_policy_unregister(&ioprio_policy);
> +}
> +
> +module_init(ioprio_init);
> +module_exit(ioprio_exit);
> diff --git a/block/blk-ioprio.h b/block/blk-ioprio.h
> new file mode 100644
> index 000000000000..a7785c2f1aea
> --- /dev/null
> +++ b/block/blk-ioprio.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _BLK_IOPRIO_H_
> +#define _BLK_IOPRIO_H_
> +
> +#include <linux/kconfig.h>
> +
> +struct request_queue;
> +
> +#ifdef CONFIG_BLK_CGROUP_IOPRIO
> +int blk_ioprio_init(struct request_queue *q);
> +#else
> +static inline int blk_ioprio_init(struct request_queue *q)
> +{
> +	return 0;
> +}
> +#endif
> +
> +#endif /* _BLK_IOPRIO_H_ */
> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index 6ac1c86f62ef..4b66d2776eda 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -946,6 +946,8 @@ static const char *rq_qos_id_to_name(enum rq_qos_id id)
>  		return "latency";
>  	case RQ_QOS_COST:
>  		return "cost";
> +	case RQ_QOS_IOPRIO:
> +		return "ioprio";
>  	}
>  	return "unknown";
>  }
> diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
> index a77afbdd472c..f000f83e0621 100644
> --- a/block/blk-rq-qos.h
> +++ b/block/blk-rq-qos.h
> @@ -17,6 +17,7 @@ enum rq_qos_id {
>  	RQ_QOS_WBT,
>  	RQ_QOS_LATENCY,
>  	RQ_QOS_COST,
> +	RQ_QOS_IOPRIO,
>  };
>  
>  struct rq_wait {


Looks good and is working as expected, here are some IOPs from a local
loopback device using this feature and the BFQ scheduler

APP     CGRP_PRIO_ON    CGRP_PRIO_OFF

IDLE    51.2K           67.2K
RT      93.6K           67.4

Reviewed by: Adam Manzanares <a.manzanares@samsung.com>
Tested by: Adam Manzanares <a.manzanares@samsung.com>