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Date: Tue, 14 Nov 2017 15:12:55 -0800
From: Shaohua Li <shli@...nel.org>
To: Tejun Heo <tj@...nel.org>
Cc: axboe@...nel.dk, linux-kernel@...r.kernel.org, kernel-team@...com,
lizefan@...wei.com, hannes@...xchg.org, cgroups@...r.kernel.org,
guro@...com
Subject: Re: [PATCH 2/7] blkcg: use percpu_ref for blkcg_gq->refcnt
On Sun, Nov 12, 2017 at 02:26:08PM -0800, Tejun Heo wrote:
> blkcg_gq->refcnt is an atomic_t. This was due to the following two
> reasons.
>
> * When blkcg_gq was added, the percpu memory allocator didn't support
> allocations from !GFP_KERNEL contexts. Because blkcg_gq's are
> created from IO issue paths, it couldn't use GFP_KERNEL allocations.
>
> * A blkcg_gq represents the connection between a cgroup and a
> request_queue. Because a in-flight bio already pins both, blkcg_gq
> didn't usually need explicit pinning, making the use of atomic_t
> acceptable albeit restrictive and fragile.
>
> Now that the percpu allocator supports !GFP_KERNEL allocations,
> there's no reason to keep using atomic_t refcnt. This will allow
> clean separation between bio and request layers helping blkcg support
> in blk-mq.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
Reviewed-by: Shaohua Li <shli@...nel.org>
> ---
> block/blk-cgroup.c | 21 ++++++++++++++++-----
> include/linux/blk-cgroup.h | 13 ++++---------
> 2 files changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 6482be5..60a4486 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -78,6 +78,7 @@ static void blkg_free(struct blkcg_gq *blkg)
>
> blkg_rwstat_exit(&blkg->stat_ios);
> blkg_rwstat_exit(&blkg->stat_bytes);
> + percpu_ref_exit(&blkg->refcnt);
> kfree(blkg);
> }
>
> @@ -89,7 +90,7 @@ static void blkg_free(struct blkcg_gq *blkg)
> * Having a reference to blkg under an rcu allows accesses to only values
> * local to groups like group stats and group rate limits.
> */
> -void __blkg_release_rcu(struct rcu_head *rcu_head)
> +static void blkg_release_rcu(struct rcu_head *rcu_head)
> {
> struct blkcg_gq *blkg = container_of(rcu_head, struct blkcg_gq, rcu_head);
>
> @@ -102,7 +103,13 @@ void __blkg_release_rcu(struct rcu_head *rcu_head)
>
> blkg_free(blkg);
> }
> -EXPORT_SYMBOL_GPL(__blkg_release_rcu);
> +
> +static void blkg_release(struct percpu_ref *refcnt)
> +{
> + struct blkcg_gq *blkg = container_of(refcnt, struct blkcg_gq, refcnt);
> +
> + call_rcu(&blkg->rcu_head, blkg_release_rcu);
> +}
>
> /**
> * blkg_alloc - allocate a blkg
> @@ -123,6 +130,11 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
> if (!blkg)
> return NULL;
>
> + if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask)) {
> + kfree(blkg);
> + return NULL;
> + }
> +
> if (blkg_rwstat_init(&blkg->stat_bytes, gfp_mask) ||
> blkg_rwstat_init(&blkg->stat_ios, gfp_mask))
> goto err_free;
> @@ -130,7 +142,6 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
> blkg->q = q;
> INIT_LIST_HEAD(&blkg->q_node);
> blkg->blkcg = blkcg;
> - atomic_set(&blkg->refcnt, 1);
>
> /* root blkg uses @q->root_rl, init rl only for !root blkgs */
> if (blkcg != &blkcg_root) {
> @@ -266,7 +277,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
> return blkg;
>
> /* @blkg failed fully initialized, use the usual release path */
> - blkg_put(blkg);
> + percpu_ref_kill(&blkg->refcnt);
> return ERR_PTR(ret);
>
> err_put_congested:
> @@ -373,7 +384,7 @@ static void blkg_destroy(struct blkcg_gq *blkg)
> * Put the reference taken at the time of creation so that when all
> * queues are gone, group can be destroyed.
> */
> - blkg_put(blkg);
> + percpu_ref_kill(&blkg->refcnt);
> }
>
> /**
> diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
> index 8bbc371..c0d4736 100644
> --- a/include/linux/blk-cgroup.h
> +++ b/include/linux/blk-cgroup.h
> @@ -19,7 +19,7 @@
> #include <linux/seq_file.h>
> #include <linux/radix-tree.h>
> #include <linux/blkdev.h>
> -#include <linux/atomic.h>
> +#include <linux/percpu-refcount.h>
>
> /* percpu_counter batch for blkg_[rw]stats, per-cpu drift doesn't matter */
> #define BLKG_STAT_CPU_BATCH (INT_MAX / 2)
> @@ -123,7 +123,7 @@ struct blkcg_gq {
> struct request_list rl;
>
> /* reference count */
> - atomic_t refcnt;
> + struct percpu_ref refcnt;
>
> /* is this blkg online? protected by both blkcg and q locks */
> bool online;
> @@ -355,21 +355,16 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
> */
> static inline void blkg_get(struct blkcg_gq *blkg)
> {
> - WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
> - atomic_inc(&blkg->refcnt);
> + percpu_ref_get(&blkg->refcnt);
> }
>
> -void __blkg_release_rcu(struct rcu_head *rcu);
> -
> /**
> * blkg_put - put a blkg reference
> * @blkg: blkg to put
> */
> static inline void blkg_put(struct blkcg_gq *blkg)
> {
> - WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
> - if (atomic_dec_and_test(&blkg->refcnt))
> - call_rcu(&blkg->rcu_head, __blkg_release_rcu);
> + percpu_ref_put(&blkg->refcnt);
> }
>
> /**
> --
> 2.9.5
>
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