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
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
---
--- /dev/null
+// 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);
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