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Make taskq_wait() block until the queue is empty

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Under Illumos taskq_wait() returns when there are no more tasks
in the queue.  This behavior differs from ZoL and FreeBSD where
taskq_wait() returns when all the tasks in the queue at the
beginning of the taskq_wait() call are complete.  New tasks
added whilst taskq_wait() is running will be ignored.

This difference in semantics makes it possible that new subtle
issues could be introduced when porting changes from Illumos.
To avoid that possibility the taskq_wait() function is being
updated such that it blocks until the queue in empty.

The previous behavior remains available through the
taskq_wait_outstanding() interface.  Note that this function
was previously called taskq_wait_all() but has been renamed
to avoid confusion.

Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #455
This commit is contained in:
Chris Dunlop 2015-05-14 12:26:51 -07:00 committed by Brian Behlendorf
parent dc5e8b7041
commit a876b0305e
3 changed files with 68 additions and 51 deletions
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2
include/sys/taskq.h
View File

@ -119,7 +119,7 @@ extern void taskq_init_ent(taskq_ent_t *);
extern taskq_t *taskq_create(const char *, int, pri_t, int, int, uint_t);
extern void taskq_destroy(taskq_t *);
extern void taskq_wait_id(taskq_t *, taskqid_t);
extern void taskq_wait_all(taskq_t *, taskqid_t);
extern void taskq_wait_outstanding(taskq_t *, taskqid_t);
extern void taskq_wait(taskq_t *);
extern int taskq_cancel_id(taskq_t *, taskqid_t);
extern int taskq_member(taskq_t *, void *);

97
module/spl/spl-taskq.c
View File

@ -327,6 +327,33 @@ taskq_find(taskq_t *tq, taskqid_t id, int *active)
return (NULL);
}
/*
* Theory for the taskq_wait_id(), taskq_wait_outstanding(), and
* taskq_wait() functions below.
*
* Taskq waiting is accomplished by tracking the lowest outstanding task
* id and the next available task id. As tasks are dispatched they are
* added to the tail of the pending, priority, or delay lists. As worker
* threads become available the tasks are removed from the heads of these
* lists and linked to the worker threads. This ensures the lists are
* kept sorted by lowest to highest task id.
*
* Therefore the lowest outstanding task id can be quickly determined by
* checking the head item from all of these lists. This value is stored
* with the taskq as the lowest id. It only needs to be recalculated when
* either the task with the current lowest id completes or is canceled.
*
* By blocking until the lowest task id exceeds the passed task id the
* taskq_wait_outstanding() function can be easily implemented. Similarly,
* by blocking until the lowest task id matches the next task id taskq_wait()
* can be implemented.
*
* Callers should be aware that when there are multiple worked threads it
* is possible for larger task ids to complete before smaller ones. Also
* when the taskq contains delay tasks with small task ids callers may
* block for a considerable length of time waiting for them to expire and
* execute.
*/
static int
taskq_wait_id_check(taskq_t *tq, taskqid_t id)
{
@ -351,34 +378,8 @@ taskq_wait_id(taskq_t *tq, taskqid_t id)
}
EXPORT_SYMBOL(taskq_wait_id);
/*
* The taskq_wait() function will block until all previously submitted
* tasks have been completed. A previously submitted task is defined as
* a task with a lower task id than the current task queue id. Note that
* all task id's are assigned monotonically at dispatch time.
*
* Waiting for all previous tasks to complete is accomplished by tracking
* the lowest outstanding task id. As tasks are dispatched they are added
* added to the tail of the pending, priority, or delay lists. And as
* worker threads become available the tasks are removed from the heads
* of these lists and linked to the worker threads. This ensures the
* lists are kept in lowest to highest task id order.
*
* Therefore the lowest outstanding task id can be quickly determined by
* checking the head item from all of these lists. This value is stored
* with the task queue as the lowest id. It only needs to be recalculated
* when either the task with the current lowest id completes or is canceled.
*
* By blocking until the lowest task id exceeds the current task id when
* the function was called we ensure all previous tasks have completed.
*
* NOTE: When there are multiple worked threads it is possible for larger
* task ids to complete before smaller ones. Conversely when the task
* queue contains delay tasks with small task ids, you may block for a
* considerable length of time waiting for them to expire and execute.
*/
static int
taskq_wait_check(taskq_t *tq, taskqid_t id)
taskq_wait_outstanding_check(taskq_t *tq, taskqid_t id)
{
int rc;
@ -389,26 +390,42 @@ taskq_wait_check(taskq_t *tq, taskqid_t id)
return (rc);
}
/*
* The taskq_wait_outstanding() function will block until all tasks with a
* lower taskqid than the passed 'id' have been completed. Note that all
* task id's are assigned monotonically at dispatch time. Zero may be
* passed for the id to indicate all tasks dispatch up to this point,
* but not after, should be waited for.
*/
void
taskq_wait_all(taskq_t *tq, taskqid_t id)
taskq_wait_outstanding(taskq_t *tq, taskqid_t id)
{
wait_event(tq->tq_wait_waitq, taskq_wait_check(tq, id));
wait_event(tq->tq_wait_waitq,
taskq_wait_outstanding_check(tq, id ? id : tq->tq_next_id - 1));
}
EXPORT_SYMBOL(taskq_wait_all);
EXPORT_SYMBOL(taskq_wait_outstanding);
static int
taskq_wait_check(taskq_t *tq)
{
int rc;
spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
rc = (tq->tq_lowest_id == tq->tq_next_id);
spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
return (rc);
}
/*
* The taskq_wait() function will block until the taskq is empty.
* This means that if a taskq re-dispatches work to itself taskq_wait()
* callers will block indefinitely.
*/
void
taskq_wait(taskq_t *tq)
{
taskqid_t id;
ASSERT(tq);
/* Wait for the largest outstanding taskqid */
spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
id = tq->tq_next_id - 1;
spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
taskq_wait_all(tq, id);
wait_event(tq->tq_wait_waitq, taskq_wait_check(tq));
}
EXPORT_SYMBOL(taskq_wait);

20
module/splat/splat-taskq.c
View File

@ -588,10 +588,10 @@ splat_taskq_test4(struct file *file, void *arg)
* next pending task as soon as it completes its current task. This
* means that tasks do not strictly complete in order in which they
* were dispatched (increasing task id). This is fine but we need to
* verify that taskq_wait_all() blocks until the passed task id and all
* lower task ids complete. We do this by dispatching the following
* verify taskq_wait_outstanding() blocks until the passed task id and
* all lower task ids complete. We do this by dispatching the following
* specific sequence of tasks each of which block for N time units.
* We then use taskq_wait_all() to unblock at specific task id and
* We then use taskq_wait_outstanding() to unblock at specific task id and
* verify the only the expected task ids have completed and in the
* correct order. The two cases of interest are:
*
@ -602,17 +602,17 @@ splat_taskq_test4(struct file *file, void *arg)
*
* The following table shows each task id and how they will be
* scheduled. Each rows represent one time unit and each column
* one of the three worker threads. The places taskq_wait_all()
* one of the three worker threads. The places taskq_wait_outstanding()
* must unblock for a specific id are identified as well as the
* task ids which must have completed and their order.
*
* +-----+ <--- taskq_wait_all(tq, 8) unblocks
* +-----+ <--- taskq_wait_outstanding(tq, 8) unblocks
* | | Required Completion Order: 1,2,4,5,3,8,6,7
* +-----+ |
* | | |
* | | +-----+
* | | | 8 |
* | | +-----+ <--- taskq_wait_all(tq, 3) unblocks
* | | +-----+ <--- taskq_wait_outstanding(tq, 3) unblocks
* | | 7 | | Required Completion Order: 1,2,4,5,3
* | +-----+ |
* | 6 | | |
@ -755,13 +755,13 @@ splat_taskq_test5_impl(struct file *file, void *arg, boolean_t prealloc)
splat_vprint(file, SPLAT_TASKQ_TEST5_NAME, "Taskq '%s' "
"waiting for taskqid %d completion\n", tq_arg.name, 3);
taskq_wait_all(tq, 3);
taskq_wait_outstanding(tq, 3);
if ((rc = splat_taskq_test_order(&tq_arg, order1)))
goto out;
splat_vprint(file, SPLAT_TASKQ_TEST5_NAME, "Taskq '%s' "
"waiting for taskqid %d completion\n", tq_arg.name, 8);
taskq_wait_all(tq, 8);
taskq_wait_outstanding(tq, 8);
rc = splat_taskq_test_order(&tq_arg, order2);
out:
@ -923,7 +923,7 @@ splat_taskq_test6_impl(struct file *file, void *arg, boolean_t prealloc)
splat_vprint(file, SPLAT_TASKQ_TEST6_NAME, "Taskq '%s' "
"waiting for taskqid %d completion\n", tq_arg.name,
SPLAT_TASKQ_ORDER_MAX);
taskq_wait_all(tq, SPLAT_TASKQ_ORDER_MAX);
taskq_wait_outstanding(tq, SPLAT_TASKQ_ORDER_MAX);
rc = splat_taskq_test_order(&tq_arg, order);
out:
@ -1030,7 +1030,7 @@ splat_taskq_test7_impl(struct file *file, void *arg, boolean_t prealloc)
if (tq_arg->flag == 0) {
splat_vprint(file, SPLAT_TASKQ_TEST7_NAME,
"Taskq '%s' waiting\n", tq_arg->name);
taskq_wait_all(tq, SPLAT_TASKQ_DEPTH_MAX);
taskq_wait_outstanding(tq, SPLAT_TASKQ_DEPTH_MAX);
}
error = (tq_arg->depth == SPLAT_TASKQ_DEPTH_MAX ? 0 : -EINVAL);