| Message ID | 20210402191638.3249835-2-schatzberg.dan@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Charge loop device i/o to issuing cgroup | expand |
On Sat, Apr 3, 2021 at 3:17 AM Dan Schatzberg <schatzberg.dan@gmail.com> 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@canonical.com> > Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com> > --- > 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@redhat.com>
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;
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@canonical.com> Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com> --- drivers/block/loop.c | 203 ++++++++++++++++++++++++++++++++++++------- drivers/block/loop.h | 12 ++- 2 files changed, 178 insertions(+), 37 deletions(-)