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blk-cgroup: reimplement basic IO stats using cgroup rstat
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blk-cgroup has been using blkg_rwstat to track basic IO stats.
Unfortunately, reading recursive stats scales badly as itinvolves
walking all descendants.  On systems with a huge number of cgroups
(dead or alive), this can lead to substantial CPU cost when reading IO
stats.

This patch reimplements basic IO stats using cgroup rstat which uses
more memory but makes recursive stat reading O(# descendants which
have been active since last reading) instead of O(# descendants).

* blk-cgroup core no longer uses sync/async stats.  Introduce new stat
  enums - BLKG_IOSTAT_{READ|WRITE|DISCARD}.

* Add blkg_iostat[_set] which encapsulates byte and io stats, last
  values for propagation delta calculation and u64_stats_sync for
  correctness on 32bit archs.

* Update the new percpu stat counters directly and implement
  blkcg_rstat_flush() to implement propagation.

* blkg_print_stat() can now bring the stats up to date by calling
  cgroup_rstat_flush() and print them instead of directly summing up
  all descendants.

* It now allocates 96 bytes per cpu.  It used to be 40 bytes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Dan Schatzberg <dschatzberg@fb.com>
Cc: Daniel Xu <dlxu@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Tejun Heo authored and Jens Axboe committed Nov 7, 2019
1 parent 8a80d5d commit f733164
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Showing 2 changed files with 142 additions and 30 deletions.
124 changes: 99 additions & 25 deletions block/blk-cgroup.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -80,8 +80,7 @@ static void blkg_free(struct blkcg_gq *blkg)
if (blkg->pd[i])
blkcg_policy[i]->pd_free_fn(blkg->pd[i]);

blkg_rwstat_exit(&blkg->stat_ios);
blkg_rwstat_exit(&blkg->stat_bytes);
free_percpu(blkg->iostat_cpu);
percpu_ref_exit(&blkg->refcnt);
kfree(blkg);
}
Expand Down Expand Up @@ -146,7 +145,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
gfp_t gfp_mask)
{
struct blkcg_gq *blkg;
int i;
int i, cpu;

/* alloc and init base part */
blkg = kzalloc_node(sizeof(*blkg), gfp_mask, q->node);
Expand All @@ -156,8 +155,8 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
goto err_free;

if (blkg_rwstat_init(&blkg->stat_bytes, gfp_mask) ||
blkg_rwstat_init(&blkg->stat_ios, gfp_mask))
blkg->iostat_cpu = alloc_percpu_gfp(struct blkg_iostat_set, gfp_mask);
if (!blkg->iostat_cpu)
goto err_free;

blkg->q = q;
Expand All @@ -167,6 +166,10 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
blkg->blkcg = blkcg;

u64_stats_init(&blkg->iostat.sync);
for_each_possible_cpu(cpu)
u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);

for (i = 0; i < BLKCG_MAX_POLS; i++) {
struct blkcg_policy *pol = blkcg_policy[i];
struct blkg_policy_data *pd;
Expand Down Expand Up @@ -393,7 +396,6 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
static void blkg_destroy(struct blkcg_gq *blkg)
{
struct blkcg *blkcg = blkg->blkcg;
struct blkcg_gq *parent = blkg->parent;
int i;

lockdep_assert_held(&blkg->q->queue_lock);
Expand All @@ -410,11 +412,6 @@ static void blkg_destroy(struct blkcg_gq *blkg)
pol->pd_offline_fn(blkg->pd[i]);
}

if (parent) {
blkg_rwstat_add_aux(&parent->stat_bytes, &blkg->stat_bytes);
blkg_rwstat_add_aux(&parent->stat_ios, &blkg->stat_ios);
}

blkg->online = false;

radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
Expand Down Expand Up @@ -464,7 +461,7 @@ static int blkcg_reset_stats(struct cgroup_subsys_state *css,
{
struct blkcg *blkcg = css_to_blkcg(css);
struct blkcg_gq *blkg;
int i;
int i, cpu;

mutex_lock(&blkcg_pol_mutex);
spin_lock_irq(&blkcg->lock);
Expand All @@ -475,8 +472,12 @@ static int blkcg_reset_stats(struct cgroup_subsys_state *css,
* anyway. If you get hit by a race, retry.
*/
hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
blkg_rwstat_reset(&blkg->stat_bytes);
blkg_rwstat_reset(&blkg->stat_ios);
for_each_possible_cpu(cpu) {
struct blkg_iostat_set *bis =
per_cpu_ptr(blkg->iostat_cpu, cpu);
memset(bis, 0, sizeof(*bis));
}
memset(&blkg->iostat, 0, sizeof(blkg->iostat));

for (i = 0; i < BLKCG_MAX_POLS; i++) {
struct blkcg_policy *pol = blkcg_policy[i];
Expand Down Expand Up @@ -840,16 +841,18 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
struct blkcg_gq *blkg;

cgroup_rstat_flush(blkcg->css.cgroup);
rcu_read_lock();

hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
struct blkg_iostat_set *bis = &blkg->iostat;
const char *dname;
char *buf;
struct blkg_rwstat_sample rwstat;
u64 rbytes, wbytes, rios, wios, dbytes, dios;
size_t size = seq_get_buf(sf, &buf), off = 0;
int i;
bool has_stats = false;
unsigned seq;

spin_lock_irq(&blkg->q->queue_lock);

Expand All @@ -868,17 +871,16 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
*/
off += scnprintf(buf+off, size-off, "%s ", dname);

blkg_rwstat_recursive_sum(blkg, NULL,
offsetof(struct blkcg_gq, stat_bytes), &rwstat);
rbytes = rwstat.cnt[BLKG_RWSTAT_READ];
wbytes = rwstat.cnt[BLKG_RWSTAT_WRITE];
dbytes = rwstat.cnt[BLKG_RWSTAT_DISCARD];
do {
seq = u64_stats_fetch_begin(&bis->sync);

blkg_rwstat_recursive_sum(blkg, NULL,
offsetof(struct blkcg_gq, stat_ios), &rwstat);
rios = rwstat.cnt[BLKG_RWSTAT_READ];
wios = rwstat.cnt[BLKG_RWSTAT_WRITE];
dios = rwstat.cnt[BLKG_RWSTAT_DISCARD];
rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
rios = bis->cur.ios[BLKG_IOSTAT_READ];
wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
} while (u64_stats_fetch_retry(&bis->sync, seq));

if (rbytes || wbytes || rios || wios) {
has_stats = true;
Expand Down Expand Up @@ -1214,6 +1216,77 @@ static int blkcg_can_attach(struct cgroup_taskset *tset)
return ret;
}

static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
{
int i;

for (i = 0; i < BLKG_IOSTAT_NR; i++) {
dst->bytes[i] = src->bytes[i];
dst->ios[i] = src->ios[i];
}
}

static void blkg_iostat_add(struct blkg_iostat *dst, struct blkg_iostat *src)
{
int i;

for (i = 0; i < BLKG_IOSTAT_NR; i++) {
dst->bytes[i] += src->bytes[i];
dst->ios[i] += src->ios[i];
}
}

static void blkg_iostat_sub(struct blkg_iostat *dst, struct blkg_iostat *src)
{
int i;

for (i = 0; i < BLKG_IOSTAT_NR; i++) {
dst->bytes[i] -= src->bytes[i];
dst->ios[i] -= src->ios[i];
}
}

static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
{
struct blkcg *blkcg = css_to_blkcg(css);
struct blkcg_gq *blkg;

rcu_read_lock();

hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
struct blkcg_gq *parent = blkg->parent;
struct blkg_iostat_set *bisc = per_cpu_ptr(blkg->iostat_cpu, cpu);
struct blkg_iostat cur, delta;
unsigned seq;

/* fetch the current per-cpu values */
do {
seq = u64_stats_fetch_begin(&bisc->sync);
blkg_iostat_set(&cur, &bisc->cur);
} while (u64_stats_fetch_retry(&bisc->sync, seq));

/* propagate percpu delta to global */
u64_stats_update_begin(&blkg->iostat.sync);
blkg_iostat_set(&delta, &cur);
blkg_iostat_sub(&delta, &bisc->last);
blkg_iostat_add(&blkg->iostat.cur, &delta);
blkg_iostat_add(&bisc->last, &delta);
u64_stats_update_end(&blkg->iostat.sync);

/* propagate global delta to parent */
if (parent) {
u64_stats_update_begin(&parent->iostat.sync);
blkg_iostat_set(&delta, &blkg->iostat.cur);
blkg_iostat_sub(&delta, &blkg->iostat.last);
blkg_iostat_add(&parent->iostat.cur, &delta);
blkg_iostat_add(&blkg->iostat.last, &delta);
u64_stats_update_end(&parent->iostat.sync);
}
}

rcu_read_unlock();
}

static void blkcg_bind(struct cgroup_subsys_state *root_css)
{
int i;
Expand Down Expand Up @@ -1246,6 +1319,7 @@ struct cgroup_subsys io_cgrp_subsys = {
.css_offline = blkcg_css_offline,
.css_free = blkcg_css_free,
.can_attach = blkcg_can_attach,
.css_rstat_flush = blkcg_rstat_flush,
.bind = blkcg_bind,
.dfl_cftypes = blkcg_files,
.legacy_cftypes = blkcg_legacy_files,
Expand Down
48 changes: 43 additions & 5 deletions include/linux/blk-cgroup.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,9 @@
*/

#include <linux/cgroup.h>
#include <linux/percpu.h>
#include <linux/percpu_counter.h>
#include <linux/u64_stats_sync.h>
#include <linux/seq_file.h>
#include <linux/radix-tree.h>
#include <linux/blkdev.h>
Expand All @@ -31,6 +33,14 @@

#ifdef CONFIG_BLK_CGROUP

enum blkg_iostat_type {
BLKG_IOSTAT_READ,
BLKG_IOSTAT_WRITE,
BLKG_IOSTAT_DISCARD,

BLKG_IOSTAT_NR,
};

enum blkg_rwstat_type {
BLKG_RWSTAT_READ,
BLKG_RWSTAT_WRITE,
Expand Down Expand Up @@ -61,6 +71,17 @@ struct blkcg {
#endif
};

struct blkg_iostat {
u64 bytes[BLKG_IOSTAT_NR];
u64 ios[BLKG_IOSTAT_NR];
};

struct blkg_iostat_set {
struct u64_stats_sync sync;
struct blkg_iostat cur;
struct blkg_iostat last;
};

/*
* blkg_[rw]stat->aux_cnt is excluded for local stats but included for
* recursive. Used to carry stats of dead children.
Expand Down Expand Up @@ -127,8 +148,8 @@ struct blkcg_gq {
/* is this blkg online? protected by both blkcg and q locks */
bool online;

struct blkg_rwstat stat_bytes;
struct blkg_rwstat stat_ios;
struct blkg_iostat_set __percpu *iostat_cpu;
struct blkg_iostat_set iostat;

struct blkg_policy_data *pd[BLKCG_MAX_POLS];

Expand Down Expand Up @@ -740,15 +761,32 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
throtl = blk_throtl_bio(q, blkg, bio);

if (!throtl) {
struct blkg_iostat_set *bis;
int rwd, cpu;

if (op_is_discard(bio->bi_opf))
rwd = BLKG_IOSTAT_DISCARD;
else if (op_is_write(bio->bi_opf))
rwd = BLKG_IOSTAT_WRITE;
else
rwd = BLKG_IOSTAT_READ;

cpu = get_cpu();
bis = per_cpu_ptr(blkg->iostat_cpu, cpu);
u64_stats_update_begin(&bis->sync);

/*
* If the bio is flagged with BIO_QUEUE_ENTERED it means this
* is a split bio and we would have already accounted for the
* size of the bio.
*/
if (!bio_flagged(bio, BIO_QUEUE_ENTERED))
blkg_rwstat_add(&blkg->stat_bytes, bio->bi_opf,
bio->bi_iter.bi_size);
blkg_rwstat_add(&blkg->stat_ios, bio->bi_opf, 1);
bis->cur.bytes[rwd] += bio->bi_iter.bi_size;
bis->cur.ios[rwd]++;

u64_stats_update_end(&bis->sync);
cgroup_rstat_updated(blkg->blkcg->css.cgroup, cpu);
put_cpu();
}

blkcg_bio_issue_init(bio);
Expand Down

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