On Sat, Apr 3, 2021 at 3:17 AM Dan Schatzberg <schatzberg.dan@xxxxxxxxx> wrote:
>
> Existing uses of loop device may have multiple cgroups reading/writing
> to the same device. Simply charging resources for I/O to the backing
> file could result in priority inversion where one cgroup gets
> synchronously blocked, holding up all other I/O to the loop device.
>
> In order to avoid this priority inversion, we use a single workqueue
> where each work item is a "struct loop_worker" which contains a queue of
> struct loop_cmds to issue. The loop device maintains a tree mapping blk
> css_id -> loop_worker. This allows each cgroup to independently make
> forward progress issuing I/O to the backing file.
>
> There is also a single queue for I/O associated with the rootcg which
> can be used in cases of extreme memory shortage where we cannot allocate
> a loop_worker.
>
> The locking for the tree and queues is fairly heavy handed - we acquire
> a per-loop-device spinlock any time either is accessed. The existing
> implementation serializes all I/O through a single thread anyways, so I
> don't believe this is any worse.
>
> Fixes-from: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Signed-off-by: Dan Schatzberg <schatzberg.dan@xxxxxxxxx>
> ---
> drivers/block/loop.c | 203 ++++++++++++++++++++++++++++++++++++-------
> drivers/block/loop.h | 12 ++-
> 2 files changed, 178 insertions(+), 37 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index d58d68f3c7cd..4750b373d4bb 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -71,7 +71,6 @@
> #include <linux/writeback.h>
> #include <linux/completion.h>
> #include <linux/highmem.h>
> -#include <linux/kthread.h>
> #include <linux/splice.h>
> #include <linux/sysfs.h>
> #include <linux/miscdevice.h>
> @@ -84,6 +83,8 @@
>
> #include <linux/uaccess.h>
>
> +#define LOOP_IDLE_WORKER_TIMEOUT (60 * HZ)
> +
> static DEFINE_IDR(loop_index_idr);
> static DEFINE_MUTEX(loop_ctl_mutex);
>
> @@ -921,27 +922,83 @@ static void loop_config_discard(struct loop_device *lo)
> q->limits.discard_alignment = 0;
> }
>
> -static void loop_unprepare_queue(struct loop_device *lo)
> -{
> - kthread_flush_worker(&lo->worker);
> - kthread_stop(lo->worker_task);
> -}
> +struct loop_worker {
> + struct rb_node rb_node;
> + struct work_struct work;
> + struct list_head cmd_list;
> + struct list_head idle_list;
> + struct loop_device *lo;
> + struct cgroup_subsys_state *css;
> + unsigned long last_ran_at;
> +};
>
> -static int loop_kthread_worker_fn(void *worker_ptr)
> -{
> - current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
> - return kthread_worker_fn(worker_ptr);
> -}
> +static void loop_workfn(struct work_struct *work);
> +static void loop_rootcg_workfn(struct work_struct *work);
> +static void loop_free_idle_workers(struct timer_list *timer);
>
> -static int loop_prepare_queue(struct loop_device *lo)
> +static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
> {
> - kthread_init_worker(&lo->worker);
> - lo->worker_task = kthread_run(loop_kthread_worker_fn,
> - &lo->worker, "loop%d", lo->lo_number);
> - if (IS_ERR(lo->worker_task))
> - return -ENOMEM;
> - set_user_nice(lo->worker_task, MIN_NICE);
> - return 0;
> + struct rb_node **node = &(lo->worker_tree.rb_node), *parent = NULL;
> + struct loop_worker *cur_worker, *worker = NULL;
> + struct work_struct *work;
> + struct list_head *cmd_list;
> +
> + spin_lock_irq(&lo->lo_work_lock);
> +
> + if (!cmd->css)
> + goto queue_work;
> +
> + node = &lo->worker_tree.rb_node;
> +
> + while (*node) {
> + parent = *node;
> + cur_worker = container_of(*node, struct loop_worker, rb_node);
> + if (cur_worker->css == cmd->css) {
> + worker = cur_worker;
> + break;
> + } else if ((long)cur_worker->css < (long)cmd->css) {
> + node = &(*node)->rb_left;
> + } else {
> + node = &(*node)->rb_right;
> + }
> + }
> + if (worker)
> + goto queue_work;
> +
> + worker = kzalloc(sizeof(struct loop_worker), GFP_NOWAIT | __GFP_NOWARN);
> + /*
> + * In the event we cannot allocate a worker, just queue on the
> + * rootcg worker
> + */
> + if (!worker)
> + goto queue_work;
> +
> + worker->css = cmd->css;
> + css_get(worker->css);
> + INIT_WORK(&worker->work, loop_workfn);
> + INIT_LIST_HEAD(&worker->cmd_list);
> + INIT_LIST_HEAD(&worker->idle_list);
> + worker->lo = lo;
> + rb_link_node(&worker->rb_node, parent, node);
> + rb_insert_color(&worker->rb_node, &lo->worker_tree);
> +queue_work:
> + if (worker) {
> + /*
> + * We need to remove from the idle list here while
> + * holding the lock so that the idle timer doesn't
> + * free the worker
> + */
> + if (!list_empty(&worker->idle_list))
> + list_del_init(&worker->idle_list);
> + work = &worker->work;
> + cmd_list = &worker->cmd_list;
> + } else {
> + work = &lo->rootcg_work;
> + cmd_list = &lo->rootcg_cmd_list;
> + }
> + list_add_tail(&cmd->list_entry, cmd_list);
> + queue_work(lo->workqueue, work);
> + spin_unlock_irq(&lo->lo_work_lock);
> }
>
> static void loop_update_rotational(struct loop_device *lo)
> @@ -1127,12 +1184,23 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> !file->f_op->write_iter)
> lo->lo_flags |= LO_FLAGS_READ_ONLY;
>
> - error = loop_prepare_queue(lo);
> - if (error)
> + lo->workqueue = alloc_workqueue("loop%d",
> + WQ_UNBOUND | WQ_FREEZABLE,
> + 0,
> + lo->lo_number);
> + if (!lo->workqueue) {
> + error = -ENOMEM;
> goto out_unlock;
> + }
>
> set_disk_ro(lo->lo_disk, (lo->lo_flags & LO_FLAGS_READ_ONLY) != 0);
>
> + INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
> + INIT_LIST_HEAD(&lo->rootcg_cmd_list);
> + INIT_LIST_HEAD(&lo->idle_worker_list);
> + lo->worker_tree = RB_ROOT;
> + timer_setup(&lo->timer, loop_free_idle_workers,
> + TIMER_DEFERRABLE);
> lo->use_dio = lo->lo_flags & LO_FLAGS_DIRECT_IO;
> lo->lo_device = bdev;
> lo->lo_backing_file = file;
> @@ -1200,6 +1268,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> int err = 0;
> bool partscan = false;
> int lo_number;
> + struct loop_worker *pos, *worker;
>
> mutex_lock(&lo->lo_mutex);
> if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
> @@ -1219,6 +1288,18 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> /* freeze request queue during the transition */
> blk_mq_freeze_queue(lo->lo_queue);
>
> + destroy_workqueue(lo->workqueue);
> + spin_lock_irq(&lo->lo_work_lock);
> + list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
> + idle_list) {
> + list_del(&worker->idle_list);
> + rb_erase(&worker->rb_node, &lo->worker_tree);
> + css_put(worker->css);
> + kfree(worker);
> + }
> + spin_unlock_irq(&lo->lo_work_lock);
> + del_timer_sync(&lo->timer);
> +
> spin_lock_irq(&lo->lo_lock);
> lo->lo_backing_file = NULL;
> spin_unlock_irq(&lo->lo_lock);
> @@ -1255,7 +1336,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>
> partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
> lo_number = lo->lo_number;
> - loop_unprepare_queue(lo);
> out_unlock:
> mutex_unlock(&lo->lo_mutex);
> if (partscan) {
> @@ -2026,7 +2106,7 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
> } else
> #endif
> cmd->css = NULL;
> - kthread_queue_work(&lo->worker, &cmd->work);
> + loop_queue_work(lo, cmd);
>
> return BLK_STS_OK;
> }
> @@ -2056,26 +2136,82 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
> }
> }
>
> -static void loop_queue_work(struct kthread_work *work)
> +static void loop_set_timer(struct loop_device *lo)
> +{
> + timer_reduce(&lo->timer, jiffies + LOOP_IDLE_WORKER_TIMEOUT);
> +}
> +
> +static void loop_process_work(struct loop_worker *worker,
> + struct list_head *cmd_list, struct loop_device *lo)
> +{
> + int orig_flags = current->flags;
> + struct loop_cmd *cmd;
> +
> + current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
> + spin_lock_irq(&lo->lo_work_lock);
> + while (!list_empty(cmd_list)) {
> + cmd = container_of(
> + cmd_list->next, struct loop_cmd, list_entry);
> + list_del(cmd_list->next);
> + spin_unlock_irq(&lo->lo_work_lock);
> +
> + loop_handle_cmd(cmd);
> + cond_resched();
> +
> + spin_lock_irq(&lo->lo_work_lock);
> + }
> +
> + /*
> + * We only add to the idle list if there are no pending cmds
> + * *and* the worker will not run again which ensures that it
> + * is safe to free any worker on the idle list
> + */
> + if (worker && !work_pending(&worker->work)) {
> + worker->last_ran_at = jiffies;
> + list_add_tail(&worker->idle_list, &lo->idle_worker_list);
> + loop_set_timer(lo);
> + }
> + spin_unlock_irq(&lo->lo_work_lock);
> + current->flags = orig_flags;
> +}
> +
> +static void loop_workfn(struct work_struct *work)
> {
> - struct loop_cmd *cmd =
> - container_of(work, struct loop_cmd, work);
> + struct loop_worker *worker =
> + container_of(work, struct loop_worker, work);
> + loop_process_work(worker, &worker->cmd_list, worker->lo);
> +}
>
> - loop_handle_cmd(cmd);
> +static void loop_rootcg_workfn(struct work_struct *work)
> +{
> + struct loop_device *lo =
> + container_of(work, struct loop_device, rootcg_work);
> + loop_process_work(NULL, &lo->rootcg_cmd_list, lo);
> }
>
> -static int loop_init_request(struct blk_mq_tag_set *set, struct request *rq,
> - unsigned int hctx_idx, unsigned int numa_node)
> +static void loop_free_idle_workers(struct timer_list *timer)
> {
> - struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
> + struct loop_device *lo = container_of(timer, struct loop_device, timer);
> + struct loop_worker *pos, *worker;
>
> - kthread_init_work(&cmd->work, loop_queue_work);
> - return 0;
> + spin_lock_irq(&lo->lo_work_lock);
> + list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
> + idle_list) {
> + if (time_is_after_jiffies(worker->last_ran_at +
> + LOOP_IDLE_WORKER_TIMEOUT))
> + break;
> + list_del(&worker->idle_list);
> + rb_erase(&worker->rb_node, &lo->worker_tree);
> + css_put(worker->css);
> + kfree(worker);
> + }
> + if (!list_empty(&lo->idle_worker_list))
> + loop_set_timer(lo);
> + spin_unlock_irq(&lo->lo_work_lock);
> }
>
> static const struct blk_mq_ops loop_mq_ops = {
> .queue_rq = loop_queue_rq,
> - .init_request = loop_init_request,
> .complete = lo_complete_rq,
> };
>
> @@ -2164,6 +2300,7 @@ static int loop_add(struct loop_device **l, int i)
> mutex_init(&lo->lo_mutex);
> lo->lo_number = i;
> spin_lock_init(&lo->lo_lock);
> + spin_lock_init(&lo->lo_work_lock);
> disk->major = LOOP_MAJOR;
> disk->first_minor = i << part_shift;
> disk->fops = &lo_fops;
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index a3c04f310672..9289c1cd6374 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -14,7 +14,6 @@
> #include <linux/blk-mq.h>
> #include <linux/spinlock.h>
> #include <linux/mutex.h>
> -#include <linux/kthread.h>
> #include <uapi/linux/loop.h>
>
> /* Possible states of device */
> @@ -54,8 +53,13 @@ struct loop_device {
>
> spinlock_t lo_lock;
> int lo_state;
> - struct kthread_worker worker;
> - struct task_struct *worker_task;
> + spinlock_t lo_work_lock;
> + struct workqueue_struct *workqueue;
> + struct work_struct rootcg_work;
> + struct list_head rootcg_cmd_list;
> + struct list_head idle_worker_list;
> + struct rb_root worker_tree;
> + struct timer_list timer;
> bool use_dio;
> bool sysfs_inited;
>
> @@ -66,7 +70,7 @@ struct loop_device {
> };
>
> struct loop_cmd {
> - struct kthread_work work;
> + struct list_head list_entry;
> bool use_aio; /* use AIO interface to handle I/O */
> atomic_t ref; /* only for aio */
> long ret;
> --
> 2.30.2
>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
--
Ming Lei
