[PATCHSET v3 block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[PATCHSET v3 block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[Tejun Heo]

Hello,

* v2[2] fixes the build failure caused by v1[1] forgetting to update bfq.

* v3 drops __acuquires/__releases() changes and updates patch descriptions.

This patchset:

* Improves blkg config helpers so that they can be used consistently for all
  the existing use cases. This also allows keeps using the same bdev open
  instance across lazy init of rq_qos policies.

* Updates iolatency so that it initializes lazily when a latency target is
  set for the first time. This avoids registering the rq_qos policy when
  iolatency is not used which removes unnecessary calls into iolat from IO
  hot paths.

and contains the following four patches:

 0001-blkcg-Drop-unnecessary-RCU-read-un-locks-from-blkg_c.patch
 0002-blkcg-Restructure-blkg_conf_prep-and-friends.patch
 0003-blk-iolatency-s-blkcg_rq_qos-iolat_rq_qos.patch
 0004-blk-iolatency-Make-initialization-lazy.patch

and is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git iolat-lazy-init-v2

diffstat follows. Thanks.

 block/bfq-cgroup.c    |    8 ++-
 block/blk-cgroup.c    |  122 ++++++++++++++++++++++++++++----------------------
 block/blk-cgroup.h    |   10 ++--
 block/blk-iocost.c    |   58 +++++++++++++----------
 block/blk-iolatency.c |   39 +++++++++++++--
 block/blk-rq-qos.h    |    2 
 block/blk-throttle.c  |   16 ++++--
 block/blk.h           |    6 --
 8 files changed, 159 insertions(+), 102 deletions(-)

[1] https://lkml.kernel.org/r/20230105002007.157497-1-tj@kernel.org
[2] https://lkml.kernel.org/r/20230105212432.289569-1-tj@kernel.org

--
tejun

[PATCH 1/4] blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()

[Tejun Heo]

Now that all RCU flavors have been combined either holding a spin lock,
disabling irq or disabling preemption implies RCU read lock, so there's no
need to use rcu_read_[un]lock() explicitly while holding queue_lock. This
shouldn't cause any behavior changes.

v2: Description updated. Leave __acquires/release on queue_lock alone.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 block/blk-cgroup.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index ce6a2b7d3dfb..7d338bc84387 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -672,12 +672,12 @@ struct block_device *blkcg_conf_open_bdev(char **inputp)
  *
  * Parse per-blkg config update from @input and initialize @ctx with the
  * result.  @ctx->blkg points to the blkg to be updated and @ctx->body the
- * part of @input following MAJ:MIN.  This function returns with RCU read
- * lock and queue lock held and must be paired with blkg_conf_finish().
+ * part of @input following MAJ:MIN.  This function returns with queue lock
+ * held and must be paired with blkg_conf_finish().
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		   char *input, struct blkg_conf_ctx *ctx)
-	__acquires(rcu) __acquires(&bdev->bd_queue->queue_lock)
+	__acquires(&bdev->bd_queue->queue_lock)
 {
 	struct block_device *bdev;
 	struct gendisk *disk;
@@ -699,7 +699,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	if (ret)
 		goto fail;
 
-	rcu_read_lock();
 	spin_lock_irq(&q->queue_lock);
 
 	if (!blkcg_policy_enabled(q, pol)) {
@@ -728,7 +727,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 
 		/* Drop locks to do new blkg allocation with GFP_KERNEL. */
 		spin_unlock_irq(&q->queue_lock);
-		rcu_read_unlock();
 
 		new_blkg = blkg_alloc(pos, disk, GFP_KERNEL);
 		if (unlikely(!new_blkg)) {
@@ -742,7 +740,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			goto fail_exit_queue;
 		}
 
-		rcu_read_lock();
 		spin_lock_irq(&q->queue_lock);
 
 		if (!blkcg_policy_enabled(q, pol)) {
@@ -778,7 +775,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	radix_tree_preload_end();
 fail_unlock:
 	spin_unlock_irq(&q->queue_lock);
-	rcu_read_unlock();
 fail_exit_queue:
 	blk_queue_exit(q);
 fail:
@@ -805,10 +801,9 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
  * with blkg_conf_prep().
  */
 void blkg_conf_finish(struct blkg_conf_ctx *ctx)
-	__releases(&ctx->bdev->bd_queue->queue_lock) __releases(rcu)
+	__releases(&ctx->bdev->bd_queue->queue_lock)
 {
 	spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
-	rcu_read_unlock();
 	blkdev_put_no_open(ctx->bdev);
 }
 EXPORT_SYMBOL_GPL(blkg_conf_finish);
-- 
2.39.0

[PATCH 2/4] blkcg: Restructure blkg_conf_prep() and friends

[Tejun Heo]

We want to support lazy init of rq-qos policies so that iolatency is enabled
lazily on configuration instead of gendisk initialization. The way blkg
config helpers are structured now is a bit awkward for that. Let's
restructure:

* blkcg_conf_open_bdev() is renamed to blkg_conf_open_bdev(). The blkcg_
  prefix was used because the bdev opening step is blkg-independent.
  However, the distinction is too subtle and confuses more than helps. Let's
  switch to blkg prefix so that it's consistent with the type and other
  helper names.

* struct blkg_conf_ctx now remembers the original input string and is always
  initialized by the new blkg_conf_init().

* blkg_conf_open_bdev() is updated to take a pointer to blkg_conf_ctx like
  blkg_conf_prep() and can be called multiple times safely. Instead of
  modifying the double pointer to input string directly,
  blkg_conf_open_bdev() now sets blkg_conf_ctx->body.

* blkg_conf_finish() is renamed to blkg_conf_exit() for symmetry and now
  must be called on all blkg_conf_ctx's which were initialized with
  blkg_conf_init().

Combined, this allows the users to either open the bdev first or do it
altogether with blkg_conf_prep() which will help implementing lazy init of
rq-qos policies.

blkg_conf_init/exit() will also be used implement synchronization against
device removal. This is necessary because iolat / iocost are configured
through cgroupfs instead of one of the files under /sys/block/DEVICE. As
cgroupfs operations aren't synchronized with block layer, the lazy init and
other configuration operations may race against device removal. This patch
makes blkg_conf_init/exit() used consistently for all cgroup-orginating
configurations making them a good place to implement explicit
synchronization.

Users are updated accordingly. No behavior change is intended by this patch.

v2: bfq wasn't updated in v1 causing a build error. Fixed.

v3: Update the description to include future use of blkg_conf_init/exit() as
    synchronization points.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Yu Kuai <yukuai1@huaweicloud.com>
---
 block/bfq-cgroup.c    |   8 ++--
 block/blk-cgroup.c    | 105 +++++++++++++++++++++++++++---------------
 block/blk-cgroup.h    |  10 ++--
 block/blk-iocost.c    |  58 +++++++++++++----------
 block/blk-iolatency.c |   8 ++--
 block/blk-throttle.c  |  16 ++++---
 6 files changed, 127 insertions(+), 78 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index a6e8da5f5cfd..97925793aee4 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -1115,9 +1115,11 @@ static ssize_t bfq_io_set_device_weight(struct kernfs_open_file *of,
 	struct bfq_group *bfqg;
 	u64 v;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_bfq, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_bfq, &ctx);
 	if (ret)
-		return ret;
+		goto out;
 
 	if (sscanf(ctx.body, "%llu", &v) == 1) {
 		/* require "default" on dfl */
@@ -1139,7 +1141,7 @@ static ssize_t bfq_io_set_device_weight(struct kernfs_open_file *of,
 		ret = 0;
 	}
 out:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 7d338bc84387..18fbdea1f756 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -626,69 +626,93 @@ u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
 EXPORT_SYMBOL_GPL(__blkg_prfill_u64);
 
 /**
- * blkcg_conf_open_bdev - parse and open bdev for per-blkg config update
- * @inputp: input string pointer
+ * blkg_conf_init - initialize a blkg_conf_ctx
+ * @ctx: blkg_conf_ctx to initialize
+ * @input: input string
  *
- * Parse the device node prefix part, MAJ:MIN, of per-blkg config update
- * from @input and get and return the matching bdev.  *@inputp is
- * updated to point past the device node prefix.  Returns an ERR_PTR()
- * value on error.
+ * Initialize @ctx which can be used to parse blkg config input string @input.
+ * Once initialized, @ctx can be used with blkg_conf_open_bdev() and
+ * blkg_conf_prep(), and must be cleaned up with blkg_conf_exit().
+ */
+void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input)
+{
+	*ctx = (struct blkg_conf_ctx){ .input = input };
+}
+EXPORT_SYMBOL_GPL(blkg_conf_init);
+
+/**
+ * blkg_conf_open_bdev - parse and open bdev for per-blkg config update
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
- * Use this function iff blkg_conf_prep() can't be used for some reason.
+ * Parse the device node prefix part, MAJ:MIN, of per-blkg config update from
+ * @ctx->input and get and store the matching bdev in @ctx->bdev. @ctx->body is
+ * set to point past the device node prefix.
+ *
+ * This function may be called multiple times on @ctx and the extra calls become
+ * NOOPs. blkg_conf_prep() implicitly calls this function. Use this function
+ * explicitly if bdev access is needed without resolving the blkcg / policy part
+ * of @ctx->input. Returns -errno on error.
  */
-struct block_device *blkcg_conf_open_bdev(char **inputp)
+int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
 {
-	char *input = *inputp;
+	char *input = ctx->input;
 	unsigned int major, minor;
 	struct block_device *bdev;
 	int key_len;
 
+	if (ctx->bdev)
+		return 0;
+
 	if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 
 	input += key_len;
 	if (!isspace(*input))
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 	input = skip_spaces(input);
 
 	bdev = blkdev_get_no_open(MKDEV(major, minor));
 	if (!bdev)
-		return ERR_PTR(-ENODEV);
+		return -ENODEV;
 	if (bdev_is_partition(bdev)) {
 		blkdev_put_no_open(bdev);
-		return ERR_PTR(-ENODEV);
+		return -ENODEV;
 	}
 
-	*inputp = input;
-	return bdev;
+	ctx->body = input;
+	ctx->bdev = bdev;
+	return 0;
 }
 
 /**
  * blkg_conf_prep - parse and prepare for per-blkg config update
  * @blkcg: target block cgroup
  * @pol: target policy
- * @input: input string
- * @ctx: blkg_conf_ctx to be filled
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
+ *
+ * Parse per-blkg config update from @ctx->input and initialize @ctx
+ * accordingly. On success, @ctx->body points to the part of @ctx->input
+ * following MAJ:MIN, @ctx->bdev points to the target block device and
+ * @ctx->blkg to the blkg being configured.
  *
- * Parse per-blkg config update from @input and initialize @ctx with the
- * result.  @ctx->blkg points to the blkg to be updated and @ctx->body the
- * part of @input following MAJ:MIN.  This function returns with queue lock
- * held and must be paired with blkg_conf_finish().
+ * blkg_conf_open_bdev() may be called on @ctx beforehand. On success, this
+ * function returns with queue lock held and must be followed by
+ * blkg_conf_exit().
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
-		   char *input, struct blkg_conf_ctx *ctx)
+		   struct blkg_conf_ctx *ctx)
 	__acquires(&bdev->bd_queue->queue_lock)
 {
-	struct block_device *bdev;
 	struct gendisk *disk;
 	struct request_queue *q;
 	struct blkcg_gq *blkg;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
-	disk = bdev->bd_disk;
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	disk = ctx->bdev->bd_disk;
 	q = disk->queue;
 
 	/*
@@ -766,9 +790,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	}
 success:
 	blk_queue_exit(q);
-	ctx->bdev = bdev;
 	ctx->blkg = blkg;
-	ctx->body = input;
 	return 0;
 
 fail_preloaded:
@@ -778,7 +800,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 fail_exit_queue:
 	blk_queue_exit(q);
 fail:
-	blkdev_put_no_open(bdev);
 	/*
 	 * If queue was bypassing, we should retry.  Do so after a
 	 * short msleep().  It isn't strictly necessary but queue
@@ -794,19 +815,27 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 EXPORT_SYMBOL_GPL(blkg_conf_prep);
 
 /**
- * blkg_conf_finish - finish up per-blkg config update
- * @ctx: blkg_conf_ctx initialized by blkg_conf_prep()
+ * blkg_conf_exit - clean up per-blkg config update
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
- * Finish up after per-blkg config update.  This function must be paired
- * with blkg_conf_prep().
+ * Clean up after per-blkg config update. This function must be called on all
+ * blkg_conf_ctx's initialized with blkg_conf_init().
  */
-void blkg_conf_finish(struct blkg_conf_ctx *ctx)
+void blkg_conf_exit(struct blkg_conf_ctx *ctx)
 	__releases(&ctx->bdev->bd_queue->queue_lock)
 {
-	spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
-	blkdev_put_no_open(ctx->bdev);
+	if (ctx->blkg) {
+		spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
+		ctx->blkg = NULL;
+	}
+
+	if (ctx->bdev) {
+		blkdev_put_no_open(ctx->bdev);
+		ctx->body = NULL;
+		ctx->bdev = NULL;
+	}
 }
-EXPORT_SYMBOL_GPL(blkg_conf_finish);
+EXPORT_SYMBOL_GPL(blkg_conf_exit);
 
 static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
 {
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 1e94e404eaa8..fe09e8b4c2a8 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -208,15 +208,17 @@ void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
 u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v);
 
 struct blkg_conf_ctx {
+	char				*input;
+	char				*body;
 	struct block_device		*bdev;
 	struct blkcg_gq			*blkg;
-	char				*body;
 };
 
-struct block_device *blkcg_conf_open_bdev(char **inputp);
+void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input);
+int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx);
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
-		   char *input, struct blkg_conf_ctx *ctx);
-void blkg_conf_finish(struct blkg_conf_ctx *ctx);
+		   struct blkg_conf_ctx *ctx);
+void blkg_conf_exit(struct blkg_conf_ctx *ctx);
 
 /**
  * bio_issue_as_root_blkg - see if this bio needs to be issued as root blkg
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 6955605629e4..22a3639a7a05 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -3091,9 +3091,11 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
 		return nbytes;
 	}
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, &ctx);
 	if (ret)
-		return ret;
+		goto err;
 
 	iocg = blkg_to_iocg(ctx.blkg);
 
@@ -3112,12 +3114,14 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
 	weight_updated(iocg, &now);
 	spin_unlock(&iocg->ioc->lock);
 
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 
 einval:
-	blkg_conf_finish(&ctx);
-	return -EINVAL;
+	ret = -EINVAL;
+err:
+	blkg_conf_exit(&ctx);
+	return ret;
 }
 
 static u64 ioc_qos_prfill(struct seq_file *sf, struct blkg_policy_data *pd,
@@ -3172,19 +3176,22 @@ static const match_table_t qos_tokens = {
 static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 			     size_t nbytes, loff_t off)
 {
-	struct block_device *bdev;
+	struct blkg_conf_ctx ctx;
 	struct gendisk *disk;
 	struct ioc *ioc;
 	u32 qos[NR_QOS_PARAMS];
 	bool enable, user;
-	char *p;
+	char *body, *p;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
+	blkg_conf_init(&ctx, input);
 
-	disk = bdev->bd_disk;
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto err;
+
+	body = ctx.body;
+	disk = ctx.bdev->bd_disk;
 	ioc = q_to_ioc(disk->queue);
 	if (!ioc) {
 		ret = blk_iocost_init(disk);
@@ -3201,7 +3208,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 	enable = ioc->enabled;
 	user = ioc->user_qos_params;
 
-	while ((p = strsep(&input, " \t\n"))) {
+	while ((p = strsep(&body, " \t\n"))) {
 		substring_t args[MAX_OPT_ARGS];
 		char buf[32];
 		int tok;
@@ -3290,7 +3297,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 	blk_mq_unquiesce_queue(disk->queue);
 	blk_mq_unfreeze_queue(disk->queue);
 
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 einval:
 	spin_unlock_irq(&ioc->lock);
@@ -3300,7 +3307,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 
 	ret = -EINVAL;
 err:
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return ret;
 }
 
@@ -3351,22 +3358,25 @@ static const match_table_t i_lcoef_tokens = {
 static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 				    size_t nbytes, loff_t off)
 {
-	struct block_device *bdev;
+	struct blkg_conf_ctx ctx;
 	struct request_queue *q;
 	struct ioc *ioc;
 	u64 u[NR_I_LCOEFS];
 	bool user;
-	char *p;
+	char *body, *p;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
+	blkg_conf_init(&ctx, input);
+
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto err;
 
-	q = bdev_get_queue(bdev);
+	body = ctx.body;
+	q = bdev_get_queue(ctx.bdev);
 	ioc = q_to_ioc(q);
 	if (!ioc) {
-		ret = blk_iocost_init(bdev->bd_disk);
+		ret = blk_iocost_init(ctx.bdev->bd_disk);
 		if (ret)
 			goto err;
 		ioc = q_to_ioc(q);
@@ -3379,7 +3389,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 	memcpy(u, ioc->params.i_lcoefs, sizeof(u));
 	user = ioc->user_cost_model;
 
-	while ((p = strsep(&input, " \t\n"))) {
+	while ((p = strsep(&body, " \t\n"))) {
 		substring_t args[MAX_OPT_ARGS];
 		char buf[32];
 		int tok;
@@ -3426,7 +3436,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 	blk_mq_unquiesce_queue(q);
 	blk_mq_unfreeze_queue(q);
 
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 
 einval:
@@ -3437,7 +3447,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 
 	ret = -EINVAL;
 err:
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return ret;
 }
 
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index ecdc10741836..3b3667f397a9 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -842,9 +842,11 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 	u64 oldval;
 	int ret;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
-		return ret;
+		goto out;
 
 	iolat = blkg_to_lat(ctx.blkg);
 	p = ctx.body;
@@ -880,7 +882,7 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 		iolatency_clear_scaling(blkg);
 	ret = 0;
 out:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 6fb5a2f9e1ee..75841d1d9bf4 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1369,9 +1369,11 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	int ret;
 	u64 v;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, &ctx);
 	if (ret)
-		return ret;
+		goto out_finish;
 
 	ret = -EINVAL;
 	if (sscanf(ctx.body, "%llu", &v) != 1)
@@ -1390,7 +1392,7 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	tg_conf_updated(tg, false);
 	ret = 0;
 out_finish:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 
@@ -1562,9 +1564,11 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 	int ret;
 	int index = of_cft(of)->private;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, &ctx);
 	if (ret)
-		return ret;
+		goto out_finish;
 
 	tg = blkg_to_tg(ctx.blkg);
 	tg_update_carryover(tg);
@@ -1663,7 +1667,7 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 		tg->td->limit_valid[LIMIT_LOW]);
 	ret = 0;
 out_finish:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 
-- 
2.39.0

[PATCH 3/4] blk-iolatency: s/blkcg_rq_qos/iolat_rq_qos/

[Tejun Heo]

The name was too generic given that there are multiple blkcg rq-qos
policies.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <josef@toxicpanda.com>
---
 block/blk-iolatency.c | 2 +-
 block/blk-rq-qos.h    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 3b3667f397a9..3601345808d2 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -976,7 +976,7 @@ static void iolatency_pd_init(struct blkg_policy_data *pd)
 {
 	struct iolatency_grp *iolat = pd_to_lat(pd);
 	struct blkcg_gq *blkg = lat_to_blkg(iolat);
-	struct rq_qos *rqos = blkcg_rq_qos(blkg->q);
+	struct rq_qos *rqos = iolat_rq_qos(blkg->q);
 	struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
 	u64 now = ktime_to_ns(ktime_get());
 	int cpu;
diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
index 1ef1f7d4bc3c..27f004fae66b 100644
--- a/block/blk-rq-qos.h
+++ b/block/blk-rq-qos.h
@@ -74,7 +74,7 @@ static inline struct rq_qos *wbt_rq_qos(struct request_queue *q)
 	return rq_qos_id(q, RQ_QOS_WBT);
 }
 
-static inline struct rq_qos *blkcg_rq_qos(struct request_queue *q)
+static inline struct rq_qos *iolat_rq_qos(struct request_queue *q)
 {
 	return rq_qos_id(q, RQ_QOS_LATENCY);
 }
-- 
2.39.0

[PATCH 4/4] blk-iolatency: Make initialization lazy

[Tejun Heo]

Other rq_qos policies such as wbt and iocost are lazy-initialized when they
are configured for the first time for the device but iolatency is
initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
init is beneficial because rq_qos policies add runtime overhead when
initialized as every IO has to walk all registered rq_qos callbacks.

This patch switches iolatency to lazy initialization too so that it only
registered its rq_qos policy when it is first configured.

Note that there is a known race condition between blkcg config file writes
and del_gendisk() and this patch makes iolatency susceptible to it by
exposing the init path to race against the deletion path. However, that
problem already exists in iocost and is being worked on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <josef@toxicpanda.com>
---
 block/blk-cgroup.c    |  8 --------
 block/blk-iolatency.c | 29 ++++++++++++++++++++++++++++-
 block/blk.h           |  6 ------
 3 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 18fbdea1f756..54e3ab68a9ac 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,7 +33,6 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
-#include "blk-rq-qos.h"
 
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
@@ -1324,14 +1323,8 @@ int blkcg_init_disk(struct gendisk *disk)
 	if (ret)
 		goto err_ioprio_exit;
 
-	ret = blk_iolatency_init(disk);
-	if (ret)
-		goto err_throtl_exit;
-
 	return 0;
 
-err_throtl_exit:
-	blk_throtl_exit(disk);
 err_ioprio_exit:
 	blk_ioprio_exit(disk);
 err_destroy_all:
@@ -1347,7 +1340,6 @@ int blkcg_init_disk(struct gendisk *disk)
 void blkcg_exit_disk(struct gendisk *disk)
 {
 	blkg_destroy_all(disk);
-	rq_qos_exit(disk->queue);
 	blk_throtl_exit(disk);
 }
 
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 3601345808d2..3484393dbc4a 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
 	}
 }
 
-int blk_iolatency_init(struct gendisk *disk)
+static int blk_iolatency_init(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
 	struct blk_iolatency *blkiolat;
@@ -830,6 +830,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
 	}
 }
 
+static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
+{
+	static DEFINE_MUTEX(init_mutex);
+	int ret;
+
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	/*
+	 * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
+	 * confuse iolat_rq_qos() test. Make the test and init atomic.
+	 */
+	mutex_lock(&init_mutex);
+
+	if (!iolat_rq_qos(ctx->bdev->bd_queue))
+		ret = blk_iolatency_init(ctx->bdev->bd_disk);
+
+	mutex_unlock(&init_mutex);
+
+	return ret;
+}
+
 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 			     size_t nbytes, loff_t off)
 {
@@ -844,6 +867,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blk_iolatency_try_init(&ctx);
+	if (ret)
+		goto out;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
 		goto out;
diff --git a/block/blk.h b/block/blk.h
index 4c3b3325219a..78f1706cddca 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -392,12 +392,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
 	return bio;
 }
 
-#ifdef CONFIG_BLK_CGROUP_IOLATENCY
-int blk_iolatency_init(struct gendisk *disk);
-#else
-static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
-#endif
-
 #ifdef CONFIG_BLK_DEV_ZONED
 void disk_free_zone_bitmaps(struct gendisk *disk);
 void disk_clear_zone_settings(struct gendisk *disk);
-- 
2.39.0

Re: [PATCHSET v3 block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[Yu Kuai]

Hi,

在 2023/01/11 6:27, Tejun Heo 写道:
> Hello,
> 
> * v2[2] fixes the build failure caused by v1[1] forgetting to update bfq.
> 
> * v3 drops __acuquires/__releases() changes and updates patch descriptions.
> 
> This patchset:
> 
> * Improves blkg config helpers so that they can be used consistently for all
>    the existing use cases. This also allows keeps using the same bdev open
>    instance across lazy init of rq_qos policies.
> 
> * Updates iolatency so that it initializes lazily when a latency target is
>    set for the first time. This avoids registering the rq_qos policy when
>    iolatency is not used which removes unnecessary calls into iolat from IO
>    hot paths.
> 

There are some rq_qos and iocost bugfix based on this patchset, can
anyone help to review this patchset?

Thanks,
Kuai

> and contains the following four patches:
> 
>   0001-blkcg-Drop-unnecessary-RCU-read-un-locks-from-blkg_c.patch
>   0002-blkcg-Restructure-blkg_conf_prep-and-friends.patch
>   0003-blk-iolatency-s-blkcg_rq_qos-iolat_rq_qos.patch
>   0004-blk-iolatency-Make-initialization-lazy.patch
> 
> and is also available in the following git branch.
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git iolat-lazy-init-v2
> 
> diffstat follows. Thanks.
> 
>   block/bfq-cgroup.c    |    8 ++-
>   block/blk-cgroup.c    |  122 ++++++++++++++++++++++++++++----------------------
>   block/blk-cgroup.h    |   10 ++--
>   block/blk-iocost.c    |   58 +++++++++++++----------
>   block/blk-iolatency.c |   39 +++++++++++++--
>   block/blk-rq-qos.h    |    2
>   block/blk-throttle.c  |   16 ++++--
>   block/blk.h           |    6 --
>   8 files changed, 159 insertions(+), 102 deletions(-)
> 
> [1] https://lkml.kernel.org/r/20230105002007.157497-1-tj@kernel.org
> [2] https://lkml.kernel.org/r/20230105212432.289569-1-tj@kernel.org
> 
> --
> tejun
> 
> .
> 

[PATCHSET v4 block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[Tejun Heo]

Hello,

* v2[2] fixes the build failure caused by v1[1] forgetting to update bfq.

* v3[3] drops __acuquires/__releases() changes and updates patch descriptions.

* v4 rebases on top of the current block/for-next.

This patchset:

* Improves blkg config helpers so that they can be used consistently for all
  the existing use cases. This also allows keeps using the same bdev open
  instance across lazy init of rq_qos policies.

* Updates iolatency so that it initializes lazily when a latency target is
  set for the first time. This avoids registering the rq_qos policy when
  iolatency is not used which removes unnecessary calls into iolat from IO
  hot paths.

and contains the following four patches:

 0001-blkcg-Drop-unnecessary-RCU-read-un-locks-from-blkg_c.patch
 0002-blkcg-Restructure-blkg_conf_prep-and-friends.patch
 0003-blk-iolatency-s-blkcg_rq_qos-iolat_rq_qos.patch
 0004-blk-iolatency-Make-initialization-lazy.patch

and is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git iolat-lazy-init-v4

diffstat follows. Thanks.

 block/bfq-cgroup.c    |    8 ++-
 block/blk-cgroup.c    |  122 ++++++++++++++++++++++++++++----------------------
 block/blk-cgroup.h    |   10 ++--
 block/blk-iocost.c    |   58 +++++++++++++----------
 block/blk-iolatency.c |   39 +++++++++++++--
 block/blk-rq-qos.h    |    2 
 block/blk-throttle.c  |   16 ++++--
 block/blk.h           |    6 --
 8 files changed, 159 insertions(+), 102 deletions(-)

[1] https://lkml.kernel.org/r/20230105002007.157497-1-tj@kernel.org
[2] https://lkml.kernel.org/r/20230105212432.289569-1-tj@kernel.org
[3] https://lkml.kernel.org/r/20230110222714.552241-1-tj@kernel.org

[PATCH 4/4] blk-iolatency: Make initialization lazy

[Tejun Heo]

Other rq_qos policies such as wbt and iocost are lazy-initialized when they
are configured for the first time for the device but iolatency is
initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
init is beneficial because rq_qos policies add runtime overhead when
initialized as every IO has to walk all registered rq_qos callbacks.

This patch switches iolatency to lazy initialization too so that it only
registered its rq_qos policy when it is first configured.

Note that there is a known race condition between blkcg config file writes
and del_gendisk() and this patch makes iolatency susceptible to it by
exposing the init path to race against the deletion path. However, that
problem already exists in iocost and is being worked on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <josef@toxicpanda.com>
---
 block/blk-cgroup.c    |  8 --------
 block/blk-iolatency.c | 29 ++++++++++++++++++++++++++++-
 block/blk.h           |  6 ------
 3 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c154b08a7e92..1c1ebeb51003 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,7 +33,6 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
-#include "blk-rq-qos.h"
 
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
@@ -1350,14 +1349,8 @@ int blkcg_init_disk(struct gendisk *disk)
 	if (ret)
 		goto err_ioprio_exit;
 
-	ret = blk_iolatency_init(disk);
-	if (ret)
-		goto err_throtl_exit;
-
 	return 0;
 
-err_throtl_exit:
-	blk_throtl_exit(disk);
 err_ioprio_exit:
 	blk_ioprio_exit(disk);
 err_destroy_all:
@@ -1373,7 +1366,6 @@ int blkcg_init_disk(struct gendisk *disk)
 void blkcg_exit_disk(struct gendisk *disk)
 {
 	blkg_destroy_all(disk);
-	rq_qos_exit(disk->queue);
 	blk_throtl_exit(disk);
 }
 
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 2560708b9109..fd5fec989e39 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
 	}
 }
 
-int blk_iolatency_init(struct gendisk *disk)
+static int blk_iolatency_init(struct gendisk *disk)
 {
 	struct blk_iolatency *blkiolat;
 	int ret;
@@ -824,6 +824,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
 	}
 }
 
+static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
+{
+	static DEFINE_MUTEX(init_mutex);
+	int ret;
+
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	/*
+	 * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
+	 * confuse iolat_rq_qos() test. Make the test and init atomic.
+	 */
+	mutex_lock(&init_mutex);
+
+	if (!iolat_rq_qos(ctx->bdev->bd_queue))
+		ret = blk_iolatency_init(ctx->bdev->bd_disk);
+
+	mutex_unlock(&init_mutex);
+
+	return ret;
+}
+
 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 			     size_t nbytes, loff_t off)
 {
@@ -838,6 +861,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blk_iolatency_try_init(&ctx);
+	if (ret)
+		goto out;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
 		goto out;
diff --git a/block/blk.h b/block/blk.h
index d65d96994a94..62fca868bc61 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -399,12 +399,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
 	return bio;
 }
 
-#ifdef CONFIG_BLK_CGROUP_IOLATENCY
-int blk_iolatency_init(struct gendisk *disk);
-#else
-static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
-#endif
-
 #ifdef CONFIG_BLK_DEV_ZONED
 void disk_free_zone_bitmaps(struct gendisk *disk);
 void disk_clear_zone_settings(struct gendisk *disk);
-- 
2.40.0

Re: [PATCH 4/4] blk-iolatency: Make initialization lazy

[Yu Kuai]

Hi,

在 2023/04/13 8:06, Tejun Heo 写道:
> Other rq_qos policies such as wbt and iocost are lazy-initialized when they
> are configured for the first time for the device but iolatency is
> initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
> init is beneficial because rq_qos policies add runtime overhead when
> initialized as every IO has to walk all registered rq_qos callbacks.
> 
> This patch switches iolatency to lazy initialization too so that it only
> registered its rq_qos policy when it is first configured.
> 
> Note that there is a known race condition between blkcg config file writes
> and del_gendisk() and this patch makes iolatency susceptible to it by
> exposing the init path to race against the deletion path. However, that
> problem already exists in iocost and is being worked on.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Cc: Josef Bacik <josef@toxicpanda.com>
> ---
>   block/blk-cgroup.c    |  8 --------
>   block/blk-iolatency.c | 29 ++++++++++++++++++++++++++++-
>   block/blk.h           |  6 ------
>   3 files changed, 28 insertions(+), 15 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index c154b08a7e92..1c1ebeb51003 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -33,7 +33,6 @@
>   #include "blk-cgroup.h"
>   #include "blk-ioprio.h"
>   #include "blk-throttle.h"
> -#include "blk-rq-qos.h"
>   
>   /*
>    * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
> @@ -1350,14 +1349,8 @@ int blkcg_init_disk(struct gendisk *disk)
>   	if (ret)
>   		goto err_ioprio_exit;
>   
> -	ret = blk_iolatency_init(disk);
> -	if (ret)
> -		goto err_throtl_exit;
> -
>   	return 0;
>   
> -err_throtl_exit:
> -	blk_throtl_exit(disk);
>   err_ioprio_exit:
>   	blk_ioprio_exit(disk);
>   err_destroy_all:
> @@ -1373,7 +1366,6 @@ int blkcg_init_disk(struct gendisk *disk)
>   void blkcg_exit_disk(struct gendisk *disk)
>   {
>   	blkg_destroy_all(disk);
> -	rq_qos_exit(disk->queue);
>   	blk_throtl_exit(disk);
>   }
>   
> diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
> index 2560708b9109..fd5fec989e39 100644
> --- a/block/blk-iolatency.c
> +++ b/block/blk-iolatency.c
> @@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
>   	}
>   }
>   
> -int blk_iolatency_init(struct gendisk *disk)
> +static int blk_iolatency_init(struct gendisk *disk)
>   {
>   	struct blk_iolatency *blkiolat;
>   	int ret;
> @@ -824,6 +824,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
>   	}
>   }
>   
> +static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
> +{
> +	static DEFINE_MUTEX(init_mutex);
> +	int ret;
> +
> +	ret = blkg_conf_open_bdev(ctx);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
> +	 * confuse iolat_rq_qos() test. Make the test and init atomic.
> +	 */
> +	mutex_lock(&init_mutex);

Can we just convert the return value from -EBUSY to 0 in this case, so
that the global lock won't be needed.

Thanks,
Kuai
> +
> +	if (!iolat_rq_qos(ctx->bdev->bd_queue))
> +		ret = blk_iolatency_init(ctx->bdev->bd_disk);
> +
> +	mutex_unlock(&init_mutex);
> +
> +	return ret;
> +}
> +
>   static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
>   			     size_t nbytes, loff_t off)
>   {
> @@ -838,6 +861,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
>   
>   	blkg_conf_init(&ctx, buf);
>   
> +	ret = blk_iolatency_try_init(&ctx);
> +	if (ret)
> +		goto out;
> +
>   	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
>   	if (ret)
>   		goto out;
> diff --git a/block/blk.h b/block/blk.h
> index d65d96994a94..62fca868bc61 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -399,12 +399,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
>   	return bio;
>   }
>   
> -#ifdef CONFIG_BLK_CGROUP_IOLATENCY
> -int blk_iolatency_init(struct gendisk *disk);
> -#else
> -static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
> -#endif
> -
>   #ifdef CONFIG_BLK_DEV_ZONED
>   void disk_free_zone_bitmaps(struct gendisk *disk);
>   void disk_clear_zone_settings(struct gendisk *disk);
> 

Re: [PATCH 4/4] blk-iolatency: Make initialization lazy

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHSET v2 block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[Tejun Heo]

Hello,

v2 fixes the build failure caused by v1[1] forgetting to update bfq.

This patchset:

* Improves blkg config helpers so that they can be used consistently for all
  the existing use cases. This also allows keeps using the same bdev open
  instance across lazy init of rq_qos policies.

* Updates iolatency so that it initializes lazily when a latency target is
  set for the first time. This avoids registering the rq_qos policy when
  iolatency is not used which removes unnecessary calls into iolat from IO
  hot paths.

and contains the following four patches:

 0001-blkcg-Drop-unnecessary-RCU-read-un-locks-from-blkg_c.patch
 0002-blkcg-Restructure-blkg_conf_prep-and-friends.patch
 0003-blk-iolatency-s-blkcg_rq_qos-iolat_rq_qos.patch
 0004-blk-iolatency-Make-initialization-lazy.patch

and is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git iolat-lazy-init-v2

diffstat follows. Thanks.

 block/bfq-cgroup.c    |    8 +++--
 block/blk-cgroup.c    |  120 ++++++++++++++++++++++++++++++++++++++++++----------------------------------
 block/blk-cgroup.h    |   10 +++---
 block/blk-iocost.c    |   58 +++++++++++++++++++++---------------
 block/blk-iolatency.c |   39 +++++++++++++++++++++---
 block/blk-rq-qos.h    |    2 -
 block/blk-throttle.c  |   16 ++++++----
 block/blk.h           |    6 ---
 8 files changed, 157 insertions(+), 102 deletions(-)

[1] https://lkml.kernel.org/r/20230105002007.157497-1-tj@kernel.org

--
tejun

[PATCH 4/4] blk-iolatency: Make initialization lazy

[Tejun Heo]

Other rq_qos policies such as wbt and iocost are lazy-initialized when they
are configured for the first time for the device but iolatency is
initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
init is beneficial because rq_qos policies add runtime overhead when
initialized as every IO has to walk all registered rq_qos callbacks.

This patch switches iolatency to lazy initialization too so that it only
registered its rq_qos policy when it is first configured.

Note that there is a known race condition between blkcg config file writes
and del_gendisk() and this patch makes iolatency susceptible to it by
exposing the init path to race against the deletion path. However, that
problem already exists in iocost and is being worked on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@lst.de>
---
 block/blk-cgroup.c    |  8 --------
 block/blk-iolatency.c | 29 ++++++++++++++++++++++++++++-
 block/blk.h           |  6 ------
 3 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d8e0625cd12d..844579aff363 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,7 +33,6 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
-#include "blk-rq-qos.h"
 
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
@@ -1322,14 +1321,8 @@ int blkcg_init_disk(struct gendisk *disk)
 	if (ret)
 		goto err_ioprio_exit;
 
-	ret = blk_iolatency_init(disk);
-	if (ret)
-		goto err_throtl_exit;
-
 	return 0;
 
-err_throtl_exit:
-	blk_throtl_exit(disk);
 err_ioprio_exit:
 	blk_ioprio_exit(disk);
 err_destroy_all:
@@ -1345,7 +1338,6 @@ int blkcg_init_disk(struct gendisk *disk)
 void blkcg_exit_disk(struct gendisk *disk)
 {
 	blkg_destroy_all(disk);
-	rq_qos_exit(disk->queue);
 	blk_throtl_exit(disk);
 }
 
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 3601345808d2..3484393dbc4a 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
 	}
 }
 
-int blk_iolatency_init(struct gendisk *disk)
+static int blk_iolatency_init(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
 	struct blk_iolatency *blkiolat;
@@ -830,6 +830,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
 	}
 }
 
+static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
+{
+	static DEFINE_MUTEX(init_mutex);
+	int ret;
+
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	/*
+	 * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
+	 * confuse iolat_rq_qos() test. Make the test and init atomic.
+	 */
+	mutex_lock(&init_mutex);
+
+	if (!iolat_rq_qos(ctx->bdev->bd_queue))
+		ret = blk_iolatency_init(ctx->bdev->bd_disk);
+
+	mutex_unlock(&init_mutex);
+
+	return ret;
+}
+
 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 			     size_t nbytes, loff_t off)
 {
@@ -844,6 +867,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blk_iolatency_try_init(&ctx);
+	if (ret)
+		goto out;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
 		goto out;
diff --git a/block/blk.h b/block/blk.h
index 4c3b3325219a..78f1706cddca 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -392,12 +392,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
 	return bio;
 }
 
-#ifdef CONFIG_BLK_CGROUP_IOLATENCY
-int blk_iolatency_init(struct gendisk *disk);
-#else
-static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
-#endif
-
 #ifdef CONFIG_BLK_DEV_ZONED
 void disk_free_zone_bitmaps(struct gendisk *disk);
 void disk_clear_zone_settings(struct gendisk *disk);
-- 
2.39.0

Re: [PATCH 4/4] blk-iolatency: Make initialization lazy

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHSET block/for-next] blkcg: Improve blkg config helpers and make iolatency init lazy

[Tejun Heo]

Hello,

* Improve blkg config helpers so that they can be used consistently for all
  the existing use cases. This also allows keeps using the same bdev open
  instance across lazy init of rq_qos policies.

* Update iolatency so that it initializes lazily when a latency target is
  set for the first time. This avoids registering the rq_qos policy when
  iolatency is not used which removes unnecessary calls into iolat from IO
  hot paths.

This patchset contains the following four patches:

 0001-blkcg-Drop-unnecessary-RCU-read-un-locks-from-blkg_c.patch
 0002-blkcg-Restructure-blkg_conf_prep-and-friends.patch
 0003-blk-iolatency-s-blkcg_rq_qos-iolat_rq_qos.patch
 0004-blk-iolatency-Make-initialization-lazy.patch

and is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git iolat-lazy-init

diffstat follows. Thanks.

 block/blk-cgroup.c    |  120 +++++++++++++++++++++++++++-----------------------
 block/blk-cgroup.h    |   10 ++--
 block/blk-iocost.c    |   58 ++++++++++++++----------
 block/blk-iolatency.c |   39 ++++++++++++++--
 block/blk-rq-qos.h    |    2 
 block/blk-throttle.c  |   16 ++++--
 block/blk.h           |    6 --
 7 files changed, 152 insertions(+), 99 deletions(-)

--
tejun

[PATCH 4/4] blk-iolatency: Make initialization lazy

[Tejun Heo]

Other rq_qos policies such as wbt and iocost are lazy-initialized when they
are configured for the first time for the device but iolatency is
initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
init is beneficial because rq_qos policies add runtime overhead when
initialized as every IO has to walk all registered rq_qos callbacks.

This patch switches iolatency to lazy initialization too so that it only
registered its rq_qos policy when it is first configured.

Note that there is a known race condition between blkcg config file writes
and del_gendisk() and this patch makes iolatency susceptible to it by
exposing the init path to race against the deletion path. However, that
problem already exists in iocost and is being worked on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@lst.de>
---
 block/blk-cgroup.c    |  8 --------
 block/blk-iolatency.c | 29 ++++++++++++++++++++++++++++-
 block/blk.h           |  6 ------
 3 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d8e0625cd12d..844579aff363 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,7 +33,6 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
-#include "blk-rq-qos.h"
 
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
@@ -1322,14 +1321,8 @@ int blkcg_init_disk(struct gendisk *disk)
 	if (ret)
 		goto err_ioprio_exit;
 
-	ret = blk_iolatency_init(disk);
-	if (ret)
-		goto err_throtl_exit;
-
 	return 0;
 
-err_throtl_exit:
-	blk_throtl_exit(disk);
 err_ioprio_exit:
 	blk_ioprio_exit(disk);
 err_destroy_all:
@@ -1345,7 +1338,6 @@ int blkcg_init_disk(struct gendisk *disk)
 void blkcg_exit_disk(struct gendisk *disk)
 {
 	blkg_destroy_all(disk);
-	rq_qos_exit(disk->queue);
 	blk_throtl_exit(disk);
 }
 
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 3601345808d2..3484393dbc4a 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
 	}
 }
 
-int blk_iolatency_init(struct gendisk *disk)
+static int blk_iolatency_init(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
 	struct blk_iolatency *blkiolat;
@@ -830,6 +830,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
 	}
 }
 
+static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
+{
+	static DEFINE_MUTEX(init_mutex);
+	int ret;
+
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	/*
+	 * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
+	 * confuse iolat_rq_qos() test. Make the test and init atomic.
+	 */
+	mutex_lock(&init_mutex);
+
+	if (!iolat_rq_qos(ctx->bdev->bd_queue))
+		ret = blk_iolatency_init(ctx->bdev->bd_disk);
+
+	mutex_unlock(&init_mutex);
+
+	return ret;
+}
+
 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 			     size_t nbytes, loff_t off)
 {
@@ -844,6 +867,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blk_iolatency_try_init(&ctx);
+	if (ret)
+		goto out;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
 		goto out;
diff --git a/block/blk.h b/block/blk.h
index 4c3b3325219a..78f1706cddca 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -392,12 +392,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
 	return bio;
 }
 
-#ifdef CONFIG_BLK_CGROUP_IOLATENCY
-int blk_iolatency_init(struct gendisk *disk);
-#else
-static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
-#endif
-
 #ifdef CONFIG_BLK_DEV_ZONED
 void disk_free_zone_bitmaps(struct gendisk *disk);
 void disk_clear_zone_settings(struct gendisk *disk);
-- 
2.39.0