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    SubjectRe: [PATCH 20/43] workqueue: reimplement work flushing using linked works
    Hello,

    On 03/01/2010 11:53 PM, Oleg Nesterov wrote:
    > On 02/26, Tejun Heo wrote:
    >>
    >> +static void move_linked_works(struct work_struct *work, struct list_head *head,
    >> + struct work_struct **nextp)
    >> +{
    >> ...
    >> + work = list_entry(work->entry.prev, struct work_struct, entry);
    >> + list_for_each_entry_safe_continue(work, n, NULL, entry) {
    >
    > list_for_each_entry_safe_from(work) ? It doesn't need to move this
    > work back.

    Yeap, that will be prettier. I used _continue there thinking continue
    will step from the current one and after finding out that it didn't, I
    rewound work not knowing about _from. Will update.

    >> @@ -680,7 +734,27 @@ static int worker_thread(void *__worker)
    >> if (kthread_should_stop())
    >> break;
    >>
    >> - run_workqueue(worker);
    >> + spin_lock_irq(&cwq->lock);
    >> +
    >> + while (!list_empty(&cwq->worklist)) {
    >> + struct work_struct *work =
    >> + list_first_entry(&cwq->worklist,
    >> + struct work_struct, entry);
    >> +
    >> + if (likely(!(*work_data_bits(work) &
    >> + WORK_STRUCT_LINKED))) {
    >> + /* optimization path, not strictly necessary */
    >> + process_one_work(worker, work);
    >> + if (unlikely(!list_empty(&worker->scheduled)))
    >> + process_scheduled_works(worker);
    >> + } else {
    >> + move_linked_works(work, &worker->scheduled,
    >> + NULL);
    >> + process_scheduled_works(worker);
    >> + }
    >> + }
    >
    > So. If the next pending work W doesn't have WORK_STRUCT_LINKED,
    > it will be executed first, then we flush ->scheduled.
    >
    > But,
    >
    >> static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
    >> - struct wq_barrier *barr, struct list_head *head)
    >> + struct wq_barrier *barr,
    >> + struct work_struct *target, struct worker *worker)
    >> {
    >> ...
    >> + /*
    >> + * If @target is currently being executed, schedule the
    >> + * barrier to the worker; otherwise, put it after @target.
    >> + */
    >> + if (worker)
    >> + head = worker->scheduled.next;
    >
    > this is the "target == current_work" case,
    >
    >> - insert_work(cwq, &barr->work, head, work_color_to_flags(WORK_NO_COLOR));
    >> + insert_work(cwq, &barr->work, head,
    >> + work_color_to_flags(WORK_NO_COLOR) | linked);
    >> }
    >
    > and in this case we put this barrier at the head of ->scheduled list.
    >
    > This means, this barrier will run after that work W, not before it?

    Yes, the barrier will run after the target work as it should.

    > Hmm. And what if there are no pending works but ->current_work == target ?
    > Again, we add the barrier to ->scheduled, but in this case worker_thread()
    > can't even notice ->scheduled is not empty because it only checks ->worklist?

    A worker always checks ->scheduled after a work is finished. IOW, if
    someone saw worker->current_work == target while holding the lock, the
    worker will check the scheduled queue after finishing the target. If
    it's not doing it somewhere, it's a bug.

    > insert_wq_barrier() also does:
    >
    > unsigned long *bits = work_data_bits(target);
    > ...
    > *bits |= WORK_STRUCT_LINKED;
    >
    > perhaps this needs atomic_long_set(), although I am not sure this really
    > matters.

    Yeah, well, work->data access is pretty messed up. At this point,
    there's no reason for atomic_long_t to begin with. No real
    atomic_long operations are used anyway. Maybe work->data started as
    proper atomic_long_t and lost its properness as it got overloaded with
    multiple things. I'm thinking about just making it a unsigned long
    and killing work_data_bits().

    Thanks.

    --
    tejun


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