[PATCH V2 2/5] blk-mq: move srcu from blk_mq_hw_ctx to request_queue

Ming Lei
2021-11-30T07:38:54Z

In case of BLK_MQ_F_BLOCKING, per-hctx srcu is used to protect dispatch
critical area. However, this srcu instance stays at the end of hctx, and
it often takes standalone cacheline, often cold.

Inside srcu_read_lock() and srcu_read_unlock(), WRITE is always done on
the indirect percpu variable which is allocated from heap instead of
being embedded, srcu->srcu_idx is read only in srcu_read_lock(). It
doesn't matter if srcu structure stays in hctx or request queue.

So switch to per-request-queue srcu for protecting dispatch, and this
way simplifies quiesce a lot, not mention quiesce is always done on the
request queue wide.

Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-core.c | 27 ++++++++++++++++++++++-----
block/blk-mq-sysfs.c | 2 --
block/blk-mq.c | 41 ++++++++++-------------------------------
block/blk-sysfs.c | 3 ++-
block/blk.h | 10 +++++++++-
block/genhd.c | 2 +-
include/linux/blk-mq.h | 8 --------
include/linux/blkdev.h | 9 +++++++++
8 files changed, 53 insertions(+), 49 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index b0660c9df852..10619fd83c1b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -66,6 +66,7 @@ DEFINE_IDA(blk_queue_ida);
* For queue allocation
*/
struct kmem_cache *blk_requestq_cachep;
+struct kmem_cache *blk_requestq_srcu_cachep;

/*
* Controlling structure to kblockd
@@ -437,21 +438,27 @@ static void blk_timeout_work(struct work_struct *work)
{
}

-struct request_queue *blk_alloc_queue(int node_id)
+struct request_queue *blk_alloc_queue(int node_id, bool alloc_srcu)
{
struct request_queue *q;
int ret;

- q = kmem_cache_alloc_node(blk_requestq_cachep,
- GFP_KERNEL | __GFP_ZERO, node_id);
+ q = kmem_cache_alloc_node(blk_get_queue_kmem_cache(alloc_srcu),
+ GFP_KERNEL | __GFP_ZERO, node_id);
if (!q)
return NULL;

+ if (alloc_srcu) {
+ blk_queue_flag_set(QUEUE_FLAG_HAS_SRCU, q);
+ if (init_srcu_struct(q->srcu) != 0)
+ goto fail_q;
+ }
+
q->last_merge = NULL;

q->id = ida_simple_get(&blk_queue_ida, 0, 0, GFP_KERNEL);
if (q->id < 0)
- goto fail_q;
+ goto fail_srcu;

ret = bioset_init(&q->bio_split, BIO_POOL_SIZE, 0, 0);
if (ret)
@@ -508,8 +515,11 @@ struct request_queue *blk_alloc_queue(int node_id)
bioset_exit(&q->bio_split);
fail_id:
ida_simple_remove(&blk_queue_ida, q->id);
+fail_srcu:
+ if (alloc_srcu)
+ cleanup_srcu_struct(q->srcu);
fail_q:
- kmem_cache_free(blk_requestq_cachep, q);
+ kmem_cache_free(blk_get_queue_kmem_cache(alloc_srcu), q);
return NULL;
}

@@ -1301,6 +1311,9 @@ int __init blk_dev_init(void)
sizeof_field(struct request, cmd_flags));
BUILD_BUG_ON(REQ_OP_BITS + REQ_FLAG_BITS > 8 *
sizeof_field(struct bio, bi_opf));
+ BUILD_BUG_ON(ALIGN(offsetof(struct request_queue, srcu),
+ __alignof__(struct request_queue)) !=
+ sizeof(struct request_queue));

/* used for unplugging and affects IO latency/throughput - HIGHPRI */
kblockd_workqueue = alloc_workqueue("kblockd",
@@ -1311,6 +1324,10 @@ int __init blk_dev_init(void)
blk_requestq_cachep = kmem_cache_create("request_queue",
sizeof(struct request_queue), 0, SLAB_PANIC, NULL);

+ blk_requestq_srcu_cachep = kmem_cache_create("request_queue_srcu",
+ sizeof(struct request_queue) +
+ sizeof(struct srcu_struct), 0, SLAB_PANIC, NULL);
+
blk_debugfs_root = debugfs_create_dir("block", NULL);

return 0;
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 253c857cba47..674786574075 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -36,8 +36,6 @@ static void blk_mq_hw_sysfs_release(struct kobject *kobj)
struct blk_mq_hw_ctx *hctx = container_of(kobj, struct blk_mq_hw_ctx,
kobj);

- if (hctx->flags & BLK_MQ_F_BLOCKING)
- cleanup_srcu_struct(hctx->srcu);
blk_free_flush_queue(hctx->fq);
sbitmap_free(&hctx->ctx_map);
free_cpumask_var(hctx->cpumask);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c5dc716b8167..a3ff671ca20e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -260,17 +260,9 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
*/
void blk_mq_wait_quiesce_done(struct request_queue *q)
{
- struct blk_mq_hw_ctx *hctx;
- unsigned int i;
- bool rcu = false;
-
- queue_for_each_hw_ctx(q, hctx, i) {
- if (hctx->flags & BLK_MQ_F_BLOCKING)
- synchronize_srcu(hctx->srcu);
- else
- rcu = true;
- }
- if (rcu)
+ if (blk_queue_has_srcu(q))
+ synchronize_srcu(q->srcu);
+ else
synchronize_rcu();
}
EXPORT_SYMBOL_GPL(blk_mq_wait_quiesce_done);
@@ -1090,9 +1082,9 @@ do { \
int srcu_idx; \
\
might_sleep(); \
- srcu_idx = srcu_read_lock(hctx->srcu); \
+ srcu_idx = srcu_read_lock(hctx->queue->srcu); \
(dispatch_ops); \
- srcu_read_unlock(hctx->srcu, srcu_idx); \
+ srcu_read_unlock(hctx->queue->srcu, srcu_idx); \
} \
} while (0)

@@ -3431,20 +3423,6 @@ static void blk_mq_exit_hw_queues(struct request_queue *q,
}
}

-static int blk_mq_hw_ctx_size(struct blk_mq_tag_set *tag_set)
-{
- int hw_ctx_size = sizeof(struct blk_mq_hw_ctx);
-
- BUILD_BUG_ON(ALIGN(offsetof(struct blk_mq_hw_ctx, srcu),
- __alignof__(struct blk_mq_hw_ctx)) !=
- sizeof(struct blk_mq_hw_ctx));
-
- if (tag_set->flags & BLK_MQ_F_BLOCKING)
- hw_ctx_size += sizeof(struct srcu_struct);
-
- return hw_ctx_size;
-}
-
static int blk_mq_init_hctx(struct request_queue *q,
struct blk_mq_tag_set *set,
struct blk_mq_hw_ctx *hctx, unsigned hctx_idx)
@@ -3482,7 +3460,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
struct blk_mq_hw_ctx *hctx;
gfp_t gfp = GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY;

- hctx = kzalloc_node(blk_mq_hw_ctx_size(set), gfp, node);
+ hctx = kzalloc_node(sizeof(struct blk_mq_hw_ctx), gfp, node);
if (!hctx)
goto fail_alloc_hctx;

@@ -3524,8 +3502,6 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
if (!hctx->fq)
goto free_bitmap;

- if (hctx->flags & BLK_MQ_F_BLOCKING)
- init_srcu_struct(hctx->srcu);
blk_mq_hctx_kobj_init(hctx);

return hctx;
@@ -3861,7 +3837,7 @@ static struct request_queue *blk_mq_init_queue_data(struct blk_mq_tag_set *set,
struct request_queue *q;
int ret;

- q = blk_alloc_queue(set->numa_node);
+ q = blk_alloc_queue(set->numa_node, set->flags & BLK_MQ_F_BLOCKING);
if (!q)
return ERR_PTR(-ENOMEM);
q->queuedata = queuedata;
@@ -4010,6 +3986,9 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
int blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
struct request_queue *q)
{
+ WARN_ON_ONCE(blk_queue_has_srcu(q) !=
+ !!(set->flags & BLK_MQ_F_BLOCKING));
+
/* mark the queue as mq asap */
q->mq_ops = set->ops;

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 4622da4bb992..3e6357321225 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -735,7 +735,8 @@ static void blk_free_queue_rcu(struct rcu_head *rcu_head)
{
struct request_queue *q = container_of(rcu_head, struct request_queue,
rcu_head);
- kmem_cache_free(blk_requestq_cachep, q);
+
+ kmem_cache_free(blk_get_queue_kmem_cache(blk_queue_has_srcu(q)), q);
}

/* Unconfigure the I/O scheduler and dissociate from the cgroup controller. */
diff --git a/block/blk.h b/block/blk.h
index a57c84654d0a..911f9f8db646 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -27,6 +27,7 @@ struct blk_flush_queue {
};

extern struct kmem_cache *blk_requestq_cachep;
+extern struct kmem_cache *blk_requestq_srcu_cachep;
extern struct kobj_type blk_queue_ktype;
extern struct ida blk_queue_ida;

@@ -428,7 +429,14 @@ int bio_add_hw_page(struct request_queue *q, struct bio *bio,
struct page *page, unsigned int len, unsigned int offset,
unsigned int max_sectors, bool *same_page);

-struct request_queue *blk_alloc_queue(int node_id);
+static inline struct kmem_cache *blk_get_queue_kmem_cache(bool srcu)
+{
+ if (srcu)
+ return blk_requestq_srcu_cachep;
+ return blk_requestq_cachep;
+}
+struct request_queue *blk_alloc_queue(int node_id, bool alloc_srcu);
+
int disk_scan_partitions(struct gendisk *disk, fmode_t mode);

int disk_alloc_events(struct gendisk *disk);
diff --git a/block/genhd.c b/block/genhd.c
index 5179a4f00fba..3c139a1b6f04 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1338,7 +1338,7 @@ struct gendisk *__blk_alloc_disk(int node, struct lock_class_key *lkclass)
struct request_queue *q;
struct gendisk *disk;

- q = blk_alloc_queue(node);
+ q = blk_alloc_queue(node, false);
if (!q)
return NULL;

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index d952c3442261..42fe97adb807 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -4,7 +4,6 @@

#include <linux/blkdev.h>
#include <linux/sbitmap.h>
-#include <linux/srcu.h>
#include <linux/lockdep.h>
#include <linux/scatterlist.h>
#include <linux/prefetch.h>
@@ -376,13 +375,6 @@ struct blk_mq_hw_ctx {
* q->unused_hctx_list.
*/
struct list_head hctx_list;
-
- /**
- * @srcu: Sleepable RCU. Use as lock when type of the hardware queue is
- * blocking (BLK_MQ_F_BLOCKING). Must be the last member - see also
- * blk_mq_hw_ctx_size().
- */
- struct srcu_struct srcu[];
};

/**
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 0a4416ef4fbf..c80cfaefc0a8 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -16,6 +16,7 @@
#include <linux/percpu-refcount.h>
#include <linux/blkzoned.h>
#include <linux/sbitmap.h>
+#include <linux/srcu.h>

struct module;
struct request_queue;
@@ -373,11 +374,18 @@ struct request_queue {
* devices that do not have multiple independent access ranges.
*/
struct blk_independent_access_ranges *ia_ranges;
+
+ /**
+ * @srcu: Sleepable RCU. Use as lock when type of the request queue
+ * is blocking (BLK_MQ_F_BLOCKING). Must be the last member
+ */
+ struct srcu_struct srcu[];
};

/* Keep blk_queue_flag_name[] in sync with the definitions below */
#define QUEUE_FLAG_STOPPED 0 /* queue is stopped */
#define QUEUE_FLAG_DYING 1 /* queue being torn down */
+#define QUEUE_FLAG_HAS_SRCU 2 /* SRCU is allocated */
#define QUEUE_FLAG_NOMERGES 3 /* disable merge attempts */
#define QUEUE_FLAG_SAME_COMP 4 /* complete on same CPU-group */
#define QUEUE_FLAG_FAIL_IO 5 /* fake timeout */
@@ -415,6 +423,7 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);

#define blk_queue_stopped(q) test_bit(QUEUE_FLAG_STOPPED, &(q)->queue_flags)
#define blk_queue_dying(q) test_bit(QUEUE_FLAG_DYING, &(q)->queue_flags)
+#define blk_queue_has_srcu(q) test_bit(QUEUE_FLAG_HAS_SRCU, &(q)->queue_flags)
#define blk_queue_dead(q) test_bit(QUEUE_FLAG_DEAD, &(q)->queue_flags)
#define blk_queue_init_done(q) test_bit(QUEUE_FLAG_INIT_DONE, &(q)->queue_flags)
#define blk_queue_nomerges(q) test_bit(QUEUE_FLAG_NOMERGES, &(q)->queue_flags)
--
2.31.1