On 11/22/24 10:25 AM, Pavel Begunkov wrote:
> 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.
If we really care about the non-lazy path as much, we can just use the
same storing scheme as we did in req->nr_tw, except in ->work_items
instead. Not a big deal imho.
--
Jens Axboe
