This is an attempt to mitigate the priority inversion problem of a high-priority blkcg issuing a sync() and being forced to wait the completion of all the writeback I/O generated by any other low-priority blkcg, causing massive latencies to processes that shouldn't be I/O-throttled at all. The idea is to save a list of blkcg's that are waiting for writeback: every time a sync() is executed the current blkcg is added to the list. Then, when I/O is throttled, if there's a blkcg waiting for writeback different than the current blkcg, no throttling is applied (we can probably refine this logic later, i.e., a better policy could be to adjust the throttling rate using the blkcg with the highest speed from the list of waiters - priority inheritance, kinda). This topic has been discussed here: https://lwn.net/ml/cgroups/20190118103127.325-1-righi.andrea@xxxxxxxxx/ But we didn't come up with any definitive solution. This patch is not a definitive solution either, but it's an attempt to continue addressing the issue and, hopefully, handle the priority inversion problem with sync() in a better way. Signed-off-by: Andrea Righi <righi.andrea@xxxxxxxxx> --- block/blk-cgroup.c | 69 ++++++++++++++++++++++++++++++++ block/blk-throttle.c | 10 +++-- fs/fs-writeback.c | 4 ++ include/linux/backing-dev-defs.h | 2 + include/linux/blk-cgroup.h | 15 +++++++ mm/backing-dev.c | 2 + 6 files changed, 99 insertions(+), 3 deletions(-) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 2bed5725aa03..d71e3cb0688d 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -1635,6 +1635,75 @@ static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now) } } +/** + * blkcg_wb_waiters_on_bdi - check for writeback waiters on a block device + * @bdi: block device to check + * + * Return true if any other blkcg is waiting for writeback on the target block + * device, false otherwise. + */ +bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi) +{ + struct blkcg *blkcg, *curr_blkcg; + bool ret = false; + + if (unlikely(!bdi)) + return false; + + rcu_read_lock(); + curr_blkcg = css_to_blkcg(task_css(current, io_cgrp_id)); + list_for_each_entry_rcu(blkcg, &bdi->cgwb_waiters, cgwb_wait_node) + if (blkcg != curr_blkcg) { + ret = true; + break; + } + rcu_read_unlock(); + + return ret; +} + +/** + * blkcg_start_wb_wait_on_bdi - add current blkcg to writeback waiters list + * @bdi: target block device + * + * Add current blkcg to the list of writeback waiters on target block device. + */ +void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi) +{ + struct blkcg *blkcg; + + rcu_read_lock(); + blkcg = css_to_blkcg(task_css(current, io_cgrp_id)); + if (blkcg) { + spin_lock(&bdi->cgwb_waiters_lock); + list_add_rcu(&blkcg->cgwb_wait_node, &bdi->cgwb_waiters); + spin_unlock(&bdi->cgwb_waiters_lock); + } + rcu_read_unlock(); +} + +/** + * blkcg_stop_wb_wait_on_bdi - remove current blkcg from writeback waiters list + * @bdi: target block device + * + * Remove current blkcg from the list of writeback waiters on target block + * device. + */ +void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi) +{ + struct blkcg *blkcg; + + rcu_read_lock(); + blkcg = css_to_blkcg(task_css(current, io_cgrp_id)); + if (blkcg) { + spin_lock(&bdi->cgwb_waiters_lock); + list_del_rcu(&blkcg->cgwb_wait_node); + spin_unlock(&bdi->cgwb_waiters_lock); + } + rcu_read_unlock(); + synchronize_rcu(); +} + /* * This is called when we want to actually walk up the hierarchy and check to * see if we need to throttle, and then actually throttle if there is some diff --git a/block/blk-throttle.c b/block/blk-throttle.c index 1b97a73d2fb1..14d9cd6e702d 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -970,9 +970,12 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio, { bool rw = bio_data_dir(bio); unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0; + struct throtl_data *td = tg->td; + struct request_queue *q = td->queue; + struct backing_dev_info *bdi = q->backing_dev_info; /* - * Currently whole state machine of group depends on first bio + * Currently whole state machine of group depends on first bio * queued in the group bio list. So one should not be calling * this function with a different bio if there are other bios * queued. @@ -981,8 +984,9 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio, bio != throtl_peek_queued(&tg->service_queue.queued[rw])); /* If tg->bps = -1, then BW is unlimited */ - if (tg_bps_limit(tg, rw) == U64_MAX && - tg_iops_limit(tg, rw) == UINT_MAX) { + if (blkcg_wb_waiters_on_bdi(bdi) || + (tg_bps_limit(tg, rw) == U64_MAX && + tg_iops_limit(tg, rw) == UINT_MAX)) { if (wait) *wait = 0; return true; diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 36855c1f8daf..13880774af3c 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -2446,6 +2446,8 @@ void sync_inodes_sb(struct super_block *sb) return; WARN_ON(!rwsem_is_locked(&sb->s_umount)); + blkcg_start_wb_wait_on_bdi(bdi); + /* protect against inode wb switch, see inode_switch_wbs_work_fn() */ bdi_down_write_wb_switch_rwsem(bdi); bdi_split_work_to_wbs(bdi, &work, false); @@ -2453,6 +2455,8 @@ void sync_inodes_sb(struct super_block *sb) bdi_up_write_wb_switch_rwsem(bdi); wait_sb_inodes(sb); + + blkcg_stop_wb_wait_on_bdi(bdi); } EXPORT_SYMBOL(sync_inodes_sb); diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index 07e02d6df5ad..095e4dd0427b 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -191,6 +191,8 @@ struct backing_dev_info { struct rb_root cgwb_congested_tree; /* their congested states */ struct mutex cgwb_release_mutex; /* protect shutdown of wb structs */ struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */ + struct list_head cgwb_waiters; /* list of all waiters for writeback */ + spinlock_t cgwb_waiters_lock; /* protect cgwb_waiters list */ #else struct bdi_writeback_congested *wb_congested; #endif diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h index 76c61318fda5..b8fe0c603ef8 100644 --- a/include/linux/blk-cgroup.h +++ b/include/linux/blk-cgroup.h @@ -56,6 +56,7 @@ struct blkcg { struct list_head all_blkcgs_node; #ifdef CONFIG_CGROUP_WRITEBACK + struct list_head cgwb_wait_node; struct list_head cgwb_list; refcount_t cgwb_refcnt; #endif @@ -454,6 +455,10 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg) blkcg_destroy_blkgs(blkcg); } +bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi); +void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi); +void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi); + #else static inline void blkcg_cgwb_get(struct blkcg *blkcg) { } @@ -464,6 +469,13 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg) blkcg_destroy_blkgs(blkcg); } +static inline bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi) +{ + return false; +} +static inline void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi) { } +static inline void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi) { } + #endif /** @@ -772,6 +784,7 @@ static inline void blkcg_bio_issue_init(struct bio *bio) static inline bool blkcg_bio_issue_check(struct request_queue *q, struct bio *bio) { + struct backing_dev_info *bdi = q->backing_dev_info; struct blkcg_gq *blkg; bool throtl = false; @@ -785,6 +798,8 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q, bio_devname(bio, b)); bio_associate_blkg(bio); } + if (blkcg_wb_waiters_on_bdi(bdi)) + bio_set_flag(bio, BIO_THROTTLED); blkg = bio->bi_blkg; diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 72e6d0c55cfa..8848d26e8bf6 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -686,10 +686,12 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi) { int ret; + INIT_LIST_HEAD(&bdi->cgwb_waiters); INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC); bdi->cgwb_congested_tree = RB_ROOT; mutex_init(&bdi->cgwb_release_mutex); init_rwsem(&bdi->wb_switch_rwsem); + spin_lock_init(&bdi->cgwb_waiters_lock); ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL); if (!ret) { -- 2.17.1
