Re: [PATCH 2/6] io_uring: replace defer task_work llist with io_wq_work_list

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/22/24 17:11, Jens Axboe wrote:
> On 11/22/24 10:07 AM, Pavel Begunkov wrote:
>> On 11/22/24 16:12, Jens Axboe wrote:
>> ...
>>>    static inline void io_req_local_work_add(struct io_kiocb *req,
>>>                         struct io_ring_ctx *ctx,
>>> -                     unsigned flags)
>>> +                     unsigned tw_flags)
>>>    {
>>> -    unsigned nr_wait, nr_tw, nr_tw_prev;
>>> -    struct llist_node *head;
>>> +    unsigned nr_tw, nr_tw_prev, nr_wait;
>>> +    unsigned long flags;
>>>          /* See comment above IO_CQ_WAKE_INIT */
>>>        BUILD_BUG_ON(IO_CQ_WAKE_FORCE <= IORING_MAX_CQ_ENTRIES);
>>>          /*
>>> -     * We don't know how many reuqests is there in the link and whether
>>> -     * they can even be queued lazily, fall back to non-lazy.
>>> +     * We don't know how many requests are in the link and whether they can
>>> +     * even be queued lazily, fall back to non-lazy.
>>>         */
>>>        if (req->flags & (REQ_F_LINK | REQ_F_HARDLINK))
>>> -        flags &= ~IOU_F_TWQ_LAZY_WAKE;
>>> +        tw_flags &= ~IOU_F_TWQ_LAZY_WAKE;
>>>    -    guard(rcu)();
>>
>> protects against ctx->task deallocation, see a comment in
>> io_ring_exit_work() -> synchronize_rcu()
> 
> Yeah that's just an editing mistake.
> 
>>> +    spin_lock_irqsave(&ctx->work_lock, flags);
>>> +    wq_list_add_tail(&req->io_task_work.work_node, &ctx->work_list);
>>> +    nr_tw_prev = ctx->work_items++;
>>
>> Is there a good reason why it changes the semantics of
>> what's stored across adds? It was assigning a corrected
>> nr_tw, this one will start heavily spamming with wake_up()
>> in some cases.
> 
> Not sure I follow, how so? nr_tw_prev will be the previous count, just
> like before. Except we won't need to dig into the list to find it, we
> have it readily available. nr_tw will be the current code, or force wake
> if needed. As before.

The problem is what it stores, not how and where. Before req->nr_tw
could've been set to IO_CQ_WAKE_FORCE, in which case following
requests are not going to attempt waking up the task, now work_items
is just a counter.

Let's say you've got a bunch of non-lazy adds coming close to each
other. The first sets IO_CQ_WAKE_FORCE and wakes the task, and
others just queue themselves in the list. Now, every single one
of them will try to wake_up() as long as ->cq_wait_nr is large
enough.

-- 
Pavel Begunkov