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Subject[PATCH 08/36] blkcg: shoot down blkio_groups on elevator switch
Date
Elevator switch may involve changes to blkcg policies.  Implement
shoot down of blkio_groups.

Combined with the previous bypass updates, the end goal is updating
blkcg core such that it can ensure that blkcg's being affected become
quiescent and don't have any per-blkg data hanging around before
commencing any policy updates. Until queues are made aware of the
policies that applies to them, as an interim step, all per-policy blkg
data will be shot down.

* blk-throtl doesn't need this change as it can't be disabled for a
live queue; however, update it anyway as the scheduled blkg
unification requires this behavior change. This means that
blk-throtl configuration will be unnecessarily lost over elevator
switch. This oddity will be removed after blkcg learns to associate
individual policies with request_queues.

* blk-throtl dosen't shoot down root_tg. This is to ease transition.
Unified blkg will always have persistent root group and not shooting
down root_tg for now eases transition to that point by avoiding
having to update td->root_tg and is safe as blk-throtl can never be
disabled

-v2: Vivek pointed out that group list is not guaranteed to be empty
on return from clear function if it raced cgroup removal and
lost. Fix it by waiting a bit and retrying. This kludge will
soon be removed once locking is updated such that blkg is never
in limbo state between blkcg and request_queue locks.

blk-throtl no longer shoots down root_tg to avoid breaking
td->root_tg.

Also, Nest queue_lock inside blkio_list_lock not the other way
around to avoid introduce possible deadlock via blkcg lock.

-v3: blkcg_clear_queue() repositioned and renamed to
blkg_destroy_all() to increase consistency with later changes.
cfq_clear_queue() updated to check q->elevator before
dereferencing it to avoid NULL dereference on not fully
initialized queues (used by later change).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
---
block/blk-cgroup.c | 34 +++++++++++++++++++++++++++++++++-
block/blk-cgroup.h | 5 ++++-
block/blk-throttle.c | 27 +++++++++++++++++++++++++--
block/cfq-iosched.c | 20 +++++++++++++++++++-
block/elevator.c | 3 +++
5 files changed, 84 insertions(+), 5 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 970a717..159aef5 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -17,8 +17,9 @@
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/slab.h>
-#include "blk-cgroup.h"
#include <linux/genhd.h>
+#include <linux/delay.h>
+#include "blk-cgroup.h"

#define MAX_KEY_LEN 100

@@ -546,6 +547,37 @@ struct blkio_group *blkiocg_lookup_group(struct blkio_cgroup *blkcg, void *key)
}
EXPORT_SYMBOL_GPL(blkiocg_lookup_group);

+void blkg_destroy_all(struct request_queue *q)
+{
+ struct blkio_policy_type *pol;
+
+ while (true) {
+ bool done = true;
+
+ spin_lock(&blkio_list_lock);
+ spin_lock_irq(q->queue_lock);
+
+ /*
+ * clear_queue_fn() might return with non-empty group list
+ * if it raced cgroup removal and lost. cgroup removal is
+ * guaranteed to make forward progress and retrying after a
+ * while is enough. This ugliness is scheduled to be
+ * removed after locking update.
+ */
+ list_for_each_entry(pol, &blkio_list, list)
+ if (!pol->ops.blkio_clear_queue_fn(q))
+ done = false;
+
+ spin_unlock_irq(q->queue_lock);
+ spin_unlock(&blkio_list_lock);
+
+ if (done)
+ break;
+
+ msleep(10); /* just some random duration I like */
+ }
+}
+
static void blkio_reset_stats_cpu(struct blkio_group *blkg)
{
struct blkio_group_stats_cpu *stats_cpu;
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 3551687..e5cfcbd 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -203,7 +203,7 @@ extern unsigned int blkcg_get_write_iops(struct blkio_cgroup *blkcg,
dev_t dev);

typedef void (blkio_unlink_group_fn) (void *key, struct blkio_group *blkg);
-
+typedef bool (blkio_clear_queue_fn)(struct request_queue *q);
typedef void (blkio_update_group_weight_fn) (void *key,
struct blkio_group *blkg, unsigned int weight);
typedef void (blkio_update_group_read_bps_fn) (void * key,
@@ -217,6 +217,7 @@ typedef void (blkio_update_group_write_iops_fn) (void *key,

struct blkio_policy_ops {
blkio_unlink_group_fn *blkio_unlink_group_fn;
+ blkio_clear_queue_fn *blkio_clear_queue_fn;
blkio_update_group_weight_fn *blkio_update_group_weight_fn;
blkio_update_group_read_bps_fn *blkio_update_group_read_bps_fn;
blkio_update_group_write_bps_fn *blkio_update_group_write_bps_fn;
@@ -233,6 +234,7 @@ struct blkio_policy_type {
/* Blkio controller policy registration */
extern void blkio_policy_register(struct blkio_policy_type *);
extern void blkio_policy_unregister(struct blkio_policy_type *);
+extern void blkg_destroy_all(struct request_queue *q);

static inline char *blkg_path(struct blkio_group *blkg)
{
@@ -249,6 +251,7 @@ struct blkio_policy_type {

static inline void blkio_policy_register(struct blkio_policy_type *blkiop) { }
static inline void blkio_policy_unregister(struct blkio_policy_type *blkiop) { }
+static inline void blkg_destroy_all(struct request_queue *q) { }

static inline char *blkg_path(struct blkio_group *blkg) { return NULL; }

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 702c0e6..3699ab4 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -989,12 +989,17 @@ throtl_destroy_tg(struct throtl_data *td, struct throtl_grp *tg)
td->nr_undestroyed_grps--;
}

-static void throtl_release_tgs(struct throtl_data *td)
+static bool throtl_release_tgs(struct throtl_data *td, bool release_root)
{
struct hlist_node *pos, *n;
struct throtl_grp *tg;
+ bool empty = true;

hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
+ /* skip root? */
+ if (!release_root && tg == td->root_tg)
+ continue;
+
/*
* If cgroup removal path got to blk_group first and removed
* it from cgroup list, then it will take care of destroying
@@ -1002,7 +1007,10 @@ static void throtl_release_tgs(struct throtl_data *td)
*/
if (!blkiocg_del_blkio_group(&tg->blkg))
throtl_destroy_tg(td, tg);
+ else
+ empty = false;
}
+ return empty;
}

/*
@@ -1029,6 +1037,20 @@ void throtl_unlink_blkio_group(void *key, struct blkio_group *blkg)
spin_unlock_irqrestore(td->queue->queue_lock, flags);
}

+static bool throtl_clear_queue(struct request_queue *q)
+{
+ lockdep_assert_held(q->queue_lock);
+
+ /*
+ * Clear tgs but leave the root one alone. This is necessary
+ * because root_tg is expected to be persistent and safe because
+ * blk-throtl can never be disabled while @q is alive. This is a
+ * kludge to prepare for unified blkg. This whole function will be
+ * removed soon.
+ */
+ return throtl_release_tgs(q->td, false);
+}
+
static void throtl_update_blkio_group_common(struct throtl_data *td,
struct throtl_grp *tg)
{
@@ -1097,6 +1119,7 @@ static void throtl_shutdown_wq(struct request_queue *q)
static struct blkio_policy_type blkio_policy_throtl = {
.ops = {
.blkio_unlink_group_fn = throtl_unlink_blkio_group,
+ .blkio_clear_queue_fn = throtl_clear_queue,
.blkio_update_group_read_bps_fn =
throtl_update_blkio_group_read_bps,
.blkio_update_group_write_bps_fn =
@@ -1282,7 +1305,7 @@ void blk_throtl_exit(struct request_queue *q)
throtl_shutdown_wq(q);

spin_lock_irq(q->queue_lock);
- throtl_release_tgs(td);
+ throtl_release_tgs(td, true);

/* If there are other groups */
if (td->nr_undestroyed_grps > 0)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 72680a6..61693d3 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1225,10 +1225,11 @@ static void cfq_destroy_cfqg(struct cfq_data *cfqd, struct cfq_group *cfqg)
cfq_put_cfqg(cfqg);
}

-static void cfq_release_cfq_groups(struct cfq_data *cfqd)
+static bool cfq_release_cfq_groups(struct cfq_data *cfqd)
{
struct hlist_node *pos, *n;
struct cfq_group *cfqg;
+ bool empty = true;

hlist_for_each_entry_safe(cfqg, pos, n, &cfqd->cfqg_list, cfqd_node) {
/*
@@ -1238,7 +1239,10 @@ static void cfq_release_cfq_groups(struct cfq_data *cfqd)
*/
if (!cfq_blkiocg_del_blkio_group(&cfqg->blkg))
cfq_destroy_cfqg(cfqd, cfqg);
+ else
+ empty = false;
}
+ return empty;
}

/*
@@ -1265,6 +1269,19 @@ static void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

+static struct elevator_type iosched_cfq;
+
+static bool cfq_clear_queue(struct request_queue *q)
+{
+ lockdep_assert_held(q->queue_lock);
+
+ /* shoot down blkgs iff the current elevator is cfq */
+ if (!q->elevator || q->elevator->type != &iosched_cfq)
+ return true;
+
+ return cfq_release_cfq_groups(q->elevator->elevator_data);
+}
+
#else /* GROUP_IOSCHED */
static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
{
@@ -3875,6 +3892,7 @@ static struct elevator_type iosched_cfq = {
static struct blkio_policy_type blkio_policy_cfq = {
.ops = {
.blkio_unlink_group_fn = cfq_unlink_blkio_group,
+ .blkio_clear_queue_fn = cfq_clear_queue,
.blkio_update_group_weight_fn = cfq_update_blkio_group_weight,
},
.plid = BLKIO_POLICY_PROP,
diff --git a/block/elevator.c b/block/elevator.c
index 0bdea0e..8c7561f 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -38,6 +38,7 @@
#include <trace/events/block.h>

#include "blk.h"
+#include "blk-cgroup.h"

static DEFINE_SPINLOCK(elv_list_lock);
static LIST_HEAD(elv_list);
@@ -894,6 +895,8 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
ioc_clear_queue(q);
spin_unlock_irq(q->queue_lock);

+ blkg_destroy_all(q);
+
/* allocate, init and register new elevator */
err = -ENOMEM;
q->elevator = elevator_alloc(q, new_e);
--
1.7.7.3


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