Re: [PATCH 4/9] blkcg: implement REQ_CGROUP_PUNT

From: Jan Kara
Date: Mon Jun 24 2019 - 12:53:47 EST

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On Sat 15-06-19 11:24:48, Tejun Heo wrote:
> When a shared kthread needs to issue a bio for a cgroup, doing so
> synchronously can lead to priority inversions as the kthread can be
> trapped waiting for that cgroup. This patch implements
> REQ_CGROUP_PUNT flag which makes submit_bio() punt the actual issuing
> to a dedicated per-blkcg work item to avoid such priority inversions.
>
> This will be used to fix priority inversions in btrfs compression and
> should be generally useful as we grow filesystem support for
> comprehensive IO control.
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Cc: Chris Mason <clm@xxxxxx>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Honza

> ---
> block/blk-cgroup.c | 53 +++++++++++++++++++++++++++++++++++++
> block/blk-core.c | 3 +++
> include/linux/backing-dev.h | 1 +
> include/linux/blk-cgroup.h | 16 ++++++++++-
> include/linux/blk_types.h | 10 +++++++
> include/linux/writeback.h | 12 ++++++---
> 6 files changed, 91 insertions(+), 4 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 07600d3c9520..48239bb93fbe 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -53,6 +53,7 @@ static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];
> static LIST_HEAD(all_blkcgs); /* protected by blkcg_pol_mutex */
>
> static bool blkcg_debug_stats = false;
> +static struct workqueue_struct *blkcg_punt_bio_wq;
>
> static bool blkcg_policy_enabled(struct request_queue *q,
> const struct blkcg_policy *pol)
> @@ -88,6 +89,8 @@ static void __blkg_release(struct rcu_head *rcu)
>
> percpu_ref_exit(&blkg->refcnt);
>
> + WARN_ON(!bio_list_empty(&blkg->async_bios));
> +
> /* release the blkcg and parent blkg refs this blkg has been holding */
> css_put(&blkg->blkcg->css);
> if (blkg->parent)
> @@ -113,6 +116,23 @@ static void blkg_release(struct percpu_ref *ref)
> call_rcu(&blkg->rcu_head, __blkg_release);
> }
>
> +static void blkg_async_bio_workfn(struct work_struct *work)
> +{
> + struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
> + async_bio_work);
> + struct bio_list bios = BIO_EMPTY_LIST;
> + struct bio *bio;
> +
> + /* as long as there are pending bios, @blkg can't go away */
> + spin_lock_bh(&blkg->async_bio_lock);
> + bio_list_merge(&bios, &blkg->async_bios);
> + bio_list_init(&blkg->async_bios);
> + spin_unlock_bh(&blkg->async_bio_lock);
> +
> + while ((bio = bio_list_pop(&bios)))
> + submit_bio(bio);
> +}
> +
> /**
> * blkg_alloc - allocate a blkg
> * @blkcg: block cgroup the new blkg is associated with
> @@ -138,6 +158,9 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
>
> blkg->q = q;
> INIT_LIST_HEAD(&blkg->q_node);
> + spin_lock_init(&blkg->async_bio_lock);
> + bio_list_init(&blkg->async_bios);
> + INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
> blkg->blkcg = blkcg;
>
> for (i = 0; i < BLKCG_MAX_POLS; i++) {
> @@ -1583,6 +1606,25 @@ void blkcg_policy_unregister(struct blkcg_policy *pol)
> }
> EXPORT_SYMBOL_GPL(blkcg_policy_unregister);
>
> +bool __blkcg_punt_bio_submit(struct bio *bio)
> +{
> + struct blkcg_gq *blkg = bio->bi_blkg;
> +
> + /* consume the flag first */
> + bio->bi_opf &= ~REQ_CGROUP_PUNT;
> +
> + /* never bounce for the root cgroup */
> + if (!blkg->parent)
> + return false;
> +
> + spin_lock_bh(&blkg->async_bio_lock);
> + bio_list_add(&blkg->async_bios, bio);
> + spin_unlock_bh(&blkg->async_bio_lock);
> +
> + queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
> + return true;
> +}
> +
> /*
> * Scale the accumulated delay based on how long it has been since we updated
> * the delay. We only call this when we are adding delay, in case it's been a
> @@ -1783,5 +1825,16 @@ void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta)
> atomic64_add(delta, &blkg->delay_nsec);
> }
>
> +static int __init blkcg_init(void)
> +{
> + blkcg_punt_bio_wq = alloc_workqueue("blkcg_punt_bio",
> + WQ_MEM_RECLAIM | WQ_FREEZABLE |
> + WQ_UNBOUND | WQ_SYSFS, 0);
> + if (!blkcg_punt_bio_wq)
> + return -ENOMEM;
> + return 0;
> +}
> +subsys_initcall(blkcg_init);
> +
> module_param(blkcg_debug_stats, bool, 0644);
> MODULE_PARM_DESC(blkcg_debug_stats, "True if you want debug stats, false if not");
> diff --git a/block/blk-core.c b/block/blk-core.c
> index a55389ba8779..5879c1ec044d 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1165,6 +1165,9 @@ EXPORT_SYMBOL_GPL(direct_make_request);
> */
> blk_qc_t submit_bio(struct bio *bio)
> {
> + if (blkcg_punt_bio_submit(bio))
> + return BLK_QC_T_NONE;
> +
> /*
> * If it's a regular read/write or a barrier with data attached,
> * go through the normal accounting stuff before submission.
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index f9b029180241..35b31d176f74 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -48,6 +48,7 @@ extern spinlock_t bdi_lock;
> extern struct list_head bdi_list;
>
> extern struct workqueue_struct *bdi_wq;
> +extern struct workqueue_struct *bdi_async_bio_wq;
>
> static inline bool wb_has_dirty_io(struct bdi_writeback *wb)
> {
> diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
> index 76c61318fda5..ffb2f88e87c6 100644
> --- a/include/linux/blk-cgroup.h
> +++ b/include/linux/blk-cgroup.h
> @@ -134,13 +134,17 @@ struct blkcg_gq {
>
> struct blkg_policy_data *pd[BLKCG_MAX_POLS];
>
> - struct rcu_head rcu_head;
> + spinlock_t async_bio_lock;
> + struct bio_list async_bios;
> + struct work_struct async_bio_work;
>
> atomic_t use_delay;
> atomic64_t delay_nsec;
> atomic64_t delay_start;
> u64 last_delay;
> int last_use;
> +
> + struct rcu_head rcu_head;
> };
>
> typedef struct blkcg_policy_data *(blkcg_pol_alloc_cpd_fn)(gfp_t gfp);
> @@ -763,6 +767,15 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
> struct bio *bio) { return false; }
> #endif
>
> +bool __blkcg_punt_bio_submit(struct bio *bio);
> +
> +static inline bool blkcg_punt_bio_submit(struct bio *bio)
> +{
> + if (bio->bi_opf & REQ_CGROUP_PUNT)
> + return __blkcg_punt_bio_submit(bio);
> + else
> + return false;
> +}
>
> static inline void blkcg_bio_issue_init(struct bio *bio)
> {
> @@ -910,6 +923,7 @@ static inline char *blkg_path(struct blkcg_gq *blkg) { return NULL; }
> static inline void blkg_get(struct blkcg_gq *blkg) { }
> static inline void blkg_put(struct blkcg_gq *blkg) { }
>
> +static inline bool blkcg_punt_bio_submit(struct bio *bio) { return false; }
> static inline void blkcg_bio_issue_init(struct bio *bio) { }
> static inline bool blkcg_bio_issue_check(struct request_queue *q,
> struct bio *bio) { return true; }
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 791fee35df88..e8b42a786315 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -321,6 +321,14 @@ enum req_flag_bits {
> __REQ_RAHEAD, /* read ahead, can fail anytime */
> __REQ_BACKGROUND, /* background IO */
> __REQ_NOWAIT, /* Don't wait if request will block */
> + /*
> + * When a shared kthread needs to issue a bio for a cgroup, doing
> + * so synchronously can lead to priority inversions as the kthread
> + * can be trapped waiting for that cgroup. CGROUP_PUNT flag makes
> + * submit_bio() punt the actual issuing to a dedicated per-blkcg
> + * work item to avoid such priority inversions.
> + */
> + __REQ_CGROUP_PUNT,
>
> /* command specific flags for REQ_OP_WRITE_ZEROES: */
> __REQ_NOUNMAP, /* do not free blocks when zeroing */
> @@ -347,6 +355,8 @@ enum req_flag_bits {
> #define REQ_RAHEAD (1ULL << __REQ_RAHEAD)
> #define REQ_BACKGROUND (1ULL << __REQ_BACKGROUND)
> #define REQ_NOWAIT (1ULL << __REQ_NOWAIT)
> +#define REQ_CGROUP_PUNT (1ULL << __REQ_CGROUP_PUNT)
> +
> #define REQ_NOUNMAP (1ULL << __REQ_NOUNMAP)
> #define REQ_HIPRI (1ULL << __REQ_HIPRI)
>
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 800ee031e88a..be602c42aab8 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -70,6 +70,7 @@ struct writeback_control {
> unsigned range_cyclic:1; /* range_start is cyclic */
> unsigned for_sync:1; /* sync(2) WB_SYNC_ALL writeback */
> unsigned no_wbc_acct:1; /* skip wbc IO accounting */
> + unsigned punt_to_cgroup:1; /* cgrp punting, see __REQ_CGROUP_PUNT */
> #ifdef CONFIG_CGROUP_WRITEBACK
> struct bdi_writeback *wb; /* wb this writeback is issued under */
> struct inode *inode; /* inode being written out */
> @@ -86,12 +87,17 @@ struct writeback_control {
>
> static inline int wbc_to_write_flags(struct writeback_control *wbc)
> {
> + int flags = 0;
> +
> + if (wbc->punt_to_cgroup)
> + flags = REQ_CGROUP_PUNT;
> +
> if (wbc->sync_mode == WB_SYNC_ALL)
> - return REQ_SYNC;
> + flags |= REQ_SYNC;
> else if (wbc->for_kupdate || wbc->for_background)
> - return REQ_BACKGROUND;
> + flags |= REQ_BACKGROUND;
>
> - return 0;
> + return flags;
> }
>
> static inline struct cgroup_subsys_state *
> --
> 2.17.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR

Next message: Logan Gunthorpe: "Re: [RFC PATCH 00/28] Removing struct page from P2PDMA"
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