[PATCH 11/15] blkcg: remove additional reference to the css

Dennis Zhou
2018-08-31T01:53:52Z

From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

The previous patch in this series removed carrying around a pointer to
the css in blkg. However, the blkg association logic still relied on
taking a reference on the css to ensure we wouldn't fail in getting a
reference for the blkg.

Here we remove the implicit dependency on the css and utilize tryget and
retry logic during association. This streamlines the three ways that
association can happen: generic, swap, writeback. They now share common
association logic with separate retry mechanisms for obtaining a copy of
the css.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
---
block/bio.c | 89 +++++++++++++++++++++++++++-----------
include/linux/blk-cgroup.h | 35 ++++-----------
include/linux/cgroup.h | 2 +
kernel/cgroup/cgroup.c | 4 +-
4 files changed, 77 insertions(+), 53 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index ec55ee810503..b792bffecce1 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1949,18 +1949,42 @@ int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)
return 0;
}

+/**
+ * __bio_associate_blkg_from_css - internal blkg association function
+ *
+ * This in the core association function that all association paths rely on.
+ * This handles -ENOMEM, but propagates -ENODEV to allow for separate retry
+ * scenarios. This takes a reference on the blkg, which is released upon
+ * freeing of the bio.
+ */
static int __bio_associate_blkg_from_css(struct bio *bio,
struct cgroup_subsys_state *css)
{
+ struct request_queue *q = bio->bi_disk->queue;
struct blkcg_gq *blkg;
+ int ret;

rcu_read_lock();

- blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_disk->queue);
+ if (!css || !css->parent) {
+ blkg = q->root_blkg;
+ } else {
+ blkg = blkg_lookup_create(css_to_blkcg(css), q);
+
+ if (IS_ERR(blkg)) {
+ ret = PTR_ERR(blkg);
+ if (ret != -ENOMEM)
+ blkg = q->root_blkg;
+ else
+ goto afc_out;
+ }
+ }

- rcu_read_unlock();
+ ret = bio_associate_blkg(bio, blkg);

- return bio_associate_blkg(bio, blkg);
+afc_out:
+ rcu_read_unlock();
+ return ret;
}

/**
@@ -1969,14 +1993,18 @@ static int __bio_associate_blkg_from_css(struct bio *bio,
* @css: target css
*
* Associate @bio with the blkg found by combining the css's blkg and the
- * request_queue of the @bio. This takes a reference on the css that will
- * be put upon freeing of @bio.
+ * request_queue of the @bio. This falls back to the queue's root_blkg if
+ * the association fails with the css.
*/
int bio_associate_blkg_from_css(struct bio *bio,
struct cgroup_subsys_state *css)
{
- css_get(css);
- return __bio_associate_blkg_from_css(bio, css);
+ if (unlikely(bio->bi_blkg))
+ return -EBUSY;
+ /* there is no retry to get another css so fallback to the root_blkg */
+ if (__bio_associate_blkg_from_css(bio, css))
+ bio_associate_blkg(bio, bio->bi_disk->queue->root_blkg);
+ return 0;
}
EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);

@@ -1987,22 +2015,35 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
* @page: the page to lookup the blkcg from
*
* Associate @bio with the blkg from @page's owning memcg and the respective
- * request_queue. This works like every other associate function wrt
- * references.
+ * request_queue. If cgroup_e_css returns NULL, fall back to the queue's
+ * root_blkg.
*
* Note: this must be called after bio has an associated device.
*/
int bio_associate_blkg_from_page(struct bio *bio, struct page *page)
{
struct cgroup_subsys_state *css;
+ int ret;

if (unlikely(bio->bi_blkg))
return -EBUSY;
if (!page->mem_cgroup)
return 0;
- css = cgroup_get_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);

- return __bio_associate_blkg_from_css(bio, css);
+ rcu_read_lock();
+
+ while (true) {
+ css = cgroup_e_css(page->mem_cgroup->css.cgroup,
+ &io_cgrp_subsys);
+
+ ret = __bio_associate_blkg_from_css(bio, css);
+ if (ret != -ENODEV)
+ break;
+ cpu_relax();
+ }
+
+ rcu_read_unlock();
+ return ret;
}
#endif /* CONFIG_MEMCG */

@@ -2012,12 +2053,12 @@ int bio_associate_blkg_from_page(struct bio *bio, struct page *page)
* @bio: target bio
*
* Associate @bio with the blkg found from the bio's css and the request_queue.
- * If one is not found, bio_lookup_blkg creates the blkg.
+ * If one is not found, bio_lookup_blkg creates the blkg. This falls back to
+ * the queue's root_blkg if association fails.
*/
int bio_associate_create_blkg(struct request_queue *q, struct bio *bio)
{
- struct blkcg *blkcg;
- struct blkcg_gq *blkg;
+ struct cgroup_subsys_state *css;
int ret = 0;

/* someone has already associated this bio with a blkg */
@@ -2026,19 +2067,19 @@ int bio_associate_create_blkg(struct request_queue *q, struct bio *bio)

rcu_read_lock();

- blkcg = css_to_blkcg(blkcg_get_css());
+ while (true) {
+ css = blkcg_css();

- if (!blkcg->css.parent) {
- ret = bio_associate_blkg(bio, q->root_blkg);
- goto assoc_out;
+ ret = __bio_associate_blkg_from_css(bio, css);
+ if (ret != -ENODEV)
+ break;
+ cpu_relax();
}

- blkg = blkg_lookup_create(blkcg, q);
- if (IS_ERR(blkg))
- blkg = q->root_blkg;
+ /* explicitly fall back to root */
+ if (unlikely(!bio->bi_blkg))
+ bio_associate_blkg(bio, q->root_blkg);

- ret = bio_associate_blkg(bio, blkg);
-assoc_out:
rcu_read_unlock();
return ret;
}
@@ -2054,8 +2095,6 @@ void bio_disassociate_task(struct bio *bio)
bio->bi_ioc = NULL;
}
if (bio->bi_blkg) {
- /* a ref is always taken on css */
- css_put(&bio_blkcg(bio)->css);
blkg_put(bio->bi_blkg);
bio->bi_blkg = NULL;
}
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 3c66154709ed..3eed491e4daa 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -233,31 +233,18 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
void blkg_conf_finish(struct blkg_conf_ctx *ctx);

/**
- * blkcg_get_css - find and get a reference to the css
+ * blk_css - find the current css
*
* Find the css associated with either the kthread or the current task.
*/
-static inline struct cgroup_subsys_state *blkcg_get_css(void)
+static inline struct cgroup_subsys_state *blkcg_css(void)
{
struct cgroup_subsys_state *css;

- rcu_read_lock();
-
css = kthread_blkcg();
- if (css) {
- css_get(css);
- } else {
- while (true) {
- css = task_css(current, io_cgrp_id);
- if (likely(css_tryget(css)))
- break;
- cpu_relax();
- }
- }
-
- rcu_read_unlock();
-
- return css;
+ if (css)
+ return css;
+ return task_css(current, io_cgrp_id);
}

static inline struct blkcg *css_to_blkcg(struct cgroup_subsys_state *css)
@@ -551,11 +538,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
rcu_read_lock();

blkcg = bio_blkcg(bio);
- if (blkcg) {
- css_get(&blkcg->css);
- } else {
- blkcg = css_to_blkcg(blkcg_get_css());
- }
+ if (!blkcg)
+ blkcg = css_to_blkcg(blkcg_css());

/* bypass blkg lookup and use @q->root_rl directly for root */
if (blkcg == &blkcg_root)
@@ -570,7 +554,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
if (unlikely(!blkg))
goto root_rl;

- blkg_get(blkg);
+ if (!blkg_try_get(blkg))
+ goto root_rl;
rcu_read_unlock();
return &blkg->rl;
root_rl:
@@ -587,8 +572,6 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
*/
static inline void blk_put_rl(struct request_list *rl)
{
- /* an additional ref is always taken for rl */
- css_put(&rl->blkg->blkcg->css);
if (rl->blkg->blkcg != &blkcg_root)
blkg_put(rl->blkg);
}
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c9fdf6f57913..0c4d56acfdca 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -93,6 +93,8 @@ extern struct css_set init_css_set;

bool css_has_online_children(struct cgroup_subsys_state *css);
struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss);
+struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgroup,
+ struct cgroup_subsys *ss);
struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgroup,
struct cgroup_subsys *ss);
struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 077370bf8964..d3fa4bdd7407 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -498,8 +498,8 @@ static struct cgroup_subsys_state *cgroup_tryget_css(struct cgroup *cgrp,
* enabled. If @ss is associated with the hierarchy @cgrp is on, this
* function is guaranteed to return non-NULL css.
*/
-static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
- struct cgroup_subsys *ss)
+struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
+ struct cgroup_subsys *ss)
{
lockdep_assert_held(&cgroup_mutex);

--
2.17.1