@@ -59,6 +59,37 @@ static struct workqueue_struct *blkcg_punt_bio_wq;
#define BLKG_DESTROY_BATCH_SIZE 64
+/*
+ * Lockless lists for tracking IO stats update
+ *
+ * New IO stats are stored in the percpu iostat_cpu within blkcg_gq (blkg).
+ * There are multiple blkg's (one for each block device) attached to each
+ * blkcg. The rstat code keeps track of which cpu has IO stats updated,
+ * but it doesn't know which blkg has the updated stats. If there are many
+ * block devices in a system, the cost of iterating all the blkg's to flush
+ * out the IO stats can be high. To reduce such overhead, a set of percpu
+ * lockless lists (lhead) per blkcg are used to track the set of recently
+ * updated iostat_cpu's since the last flush. An iostat_cpu will be put
+ * onto the lockless list on the update side [blk_cgroup_bio_start()] if
+ * not there yet and then removed when being flushed [blkcg_rstat_flush()].
+ * References to blkg are gotten and then put back in the process to
+ * protect against blkg removal.
+ *
+ * Return: 0 if successful or -ENOMEM if allocation fails.
+ */
+static int init_blkcg_llists(struct blkcg *blkcg)
+{
+ int cpu;
+
+ blkcg->lhead = alloc_percpu_gfp(struct llist_head, GFP_KERNEL);
+ if (!blkcg->lhead)
+ return -ENOMEM;
+
+ for_each_possible_cpu(cpu)
+ init_llist_head(per_cpu_ptr(blkcg->lhead, cpu));
+ return 0;
+}
+
/**
* blkcg_css - find the current css
*
@@ -236,8 +267,10 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
blkg->blkcg = blkcg;
u64_stats_init(&blkg->iostat.sync);
- for_each_possible_cpu(cpu)
+ for_each_possible_cpu(cpu) {
u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
+ per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
+ }
for (i = 0; i < BLKCG_MAX_POLS; i++) {
struct blkcg_policy *pol = blkcg_policy[i];
@@ -827,7 +860,9 @@ static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur,
static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
{
struct blkcg *blkcg = css_to_blkcg(css);
- struct blkcg_gq *blkg;
+ struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
+ struct llist_node *lnode;
+ struct blkg_iostat_set *bisc, *next_bisc;
/* Root-level stats are sourced from system-wide IO stats */
if (!cgroup_parent(css->cgroup))
@@ -835,12 +870,21 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
rcu_read_lock();
- hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
+ lnode = llist_del_all(lhead);
+ if (!lnode)
+ goto out;
+
+ /*
+ * Iterate only the iostat_cpu's queued in the lockless list.
+ */
+ llist_for_each_entry_safe(bisc, next_bisc, lnode, lnode) {
+ struct blkcg_gq *blkg = bisc->blkg;
struct blkcg_gq *parent = blkg->parent;
- struct blkg_iostat_set *bisc = per_cpu_ptr(blkg->iostat_cpu, cpu);
struct blkg_iostat cur;
unsigned int seq;
+ WRITE_ONCE(bisc->lqueued, false);
+
/* fetch the current per-cpu values */
do {
seq = u64_stats_fetch_begin(&bisc->sync);
@@ -853,8 +897,10 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
if (parent && parent->parent)
blkcg_iostat_update(parent, &blkg->iostat.cur,
&blkg->iostat.last);
+ percpu_ref_put(&blkg->refcnt);
}
+out:
rcu_read_unlock();
}
@@ -1132,6 +1178,7 @@ static void blkcg_css_free(struct cgroup_subsys_state *css)
mutex_unlock(&blkcg_pol_mutex);
+ free_percpu(blkcg->lhead);
kfree(blkcg);
}
@@ -1151,6 +1198,9 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
goto unlock;
}
+ if (init_blkcg_llists(blkcg))
+ goto free_blkcg;
+
for (i = 0; i < BLKCG_MAX_POLS ; i++) {
struct blkcg_policy *pol = blkcg_policy[i];
struct blkcg_policy_data *cpd;
@@ -1191,7 +1241,8 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
for (i--; i >= 0; i--)
if (blkcg->cpd[i])
blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
-
+ free_percpu(blkcg->lhead);
+free_blkcg:
if (blkcg != &blkcg_root)
kfree(blkcg);
unlock:
@@ -1939,6 +1990,7 @@ static int blk_cgroup_io_type(struct bio *bio)
void blk_cgroup_bio_start(struct bio *bio)
{
+ struct blkcg *blkcg = bio->bi_blkg->blkcg;
int rwd = blk_cgroup_io_type(bio), cpu;
struct blkg_iostat_set *bis;
unsigned long flags;
@@ -1957,9 +2009,21 @@ void blk_cgroup_bio_start(struct bio *bio)
}
bis->cur.ios[rwd]++;
+ /*
+ * If the iostat_cpu isn't in a lockless list, put it into the
+ * list to indicate that a stat update is pending.
+ */
+ if (!READ_ONCE(bis->lqueued)) {
+ struct llist_head *lhead = this_cpu_ptr(blkcg->lhead);
+
+ llist_add(&bis->lnode, lhead);
+ WRITE_ONCE(bis->lqueued, true);
+ percpu_ref_get(&bis->blkg->refcnt);
+ }
+
u64_stats_update_end_irqrestore(&bis->sync, flags);
if (cgroup_subsys_on_dfl(io_cgrp_subsys))
- cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu);
+ cgroup_rstat_updated(blkcg->css.cgroup, cpu);
put_cpu();
}
@@ -18,6 +18,7 @@
#include <linux/cgroup.h>
#include <linux/kthread.h>
#include <linux/blk-mq.h>
+#include <linux/llist.h>
struct blkcg_gq;
struct blkg_policy_data;
@@ -43,6 +44,9 @@ struct blkg_iostat {
struct blkg_iostat_set {
struct u64_stats_sync sync;
+ struct blkcg_gq *blkg;
+ struct llist_node lnode;
+ int lqueued; /* queued in llist */
struct blkg_iostat cur;
struct blkg_iostat last;
};
@@ -97,6 +101,12 @@ struct blkcg {
struct blkcg_policy_data *cpd[BLKCG_MAX_POLS];
struct list_head all_blkcgs_node;
+
+ /*
+ * List of updated percpu blkg_iostat_set's since the last flush.
+ */
+ struct llist_head __percpu *lhead;
+
#ifdef CONFIG_BLK_CGROUP_FC_APPID
char fc_app_id[FC_APPID_LEN];
#endif