Re: [PATCH RFC v4 1/3] block: add BIO_COMPLETE_IN_TASK for task-context completion

[Tal Zussman <tz2294@columbia.edu>]

On 3/25/26 4:14 PM, Jens Axboe wrote:
> On 3/25/26 12:43 PM, Tal Zussman wrote:
>> +static void bio_complete_work_fn(struct work_struct *w)
>> +{
>> +	struct bio_complete_batch *batch;
>> +	struct bio_list list;
>> +
>> +again:
>> +	local_lock_irq(&bio_complete_batch.lock);
>> +	batch = this_cpu_ptr(&bio_complete_batch);
>> +	list = batch->list;
>> +	bio_list_init(&batch->list);
>> +	local_unlock_irq(&bio_complete_batch.lock);
>> +
>> +	while (!bio_list_empty(&list)) {
>> +		struct bio *bio = bio_list_pop(&list);
>> +		bio->bi_end_io(bio);
>> +	}
>> +
>> +	local_lock_irq(&bio_complete_batch.lock);
>> +	batch = this_cpu_ptr(&bio_complete_batch);
>> +	if (!bio_list_empty(&batch->list)) {
>> +		local_unlock_irq(&bio_complete_batch.lock);
>> +
>> +		if (!need_resched())
>> +			goto again;
>> +
>> +		schedule_work_on(smp_processor_id(), &batch->work);
>> +		return;
>> +	}
>> +	local_unlock_irq(&bio_complete_batch.lock);
>> +}
> 
> bool looped = false;
> 
> do {
> 	if (looped && need_resched()) {
>      		schedule_work_on(smp_processor_id(), &batch->work);
> 		break;
> 	}
> 
> 	local_lock_irq(&bio_complete_batch.lock);
> 	batch = this_cpu_ptr(&bio_complete_batch);
> 	list = batch->list;
> 	bio_list_init(&batch->list);
> 	local_unlock_irq(&bio_complete_batch.lock);
> 
> 	if (bio_list_empty(&list))
> 		break;
> 
> 	do {
> 		struct bio *bio = bio_list_pop(&list);
> 		bio->bi_end_io(bio);
> 	} while (!bio_list_empty(&list));
> 	looped = true;
> } while (1);
> 
> would be a lot easier to read, and avoid needing the list manipulation
> included twice.

Yep, that looks cleaner. Although do we really need the looped variable?
Can't we just move the need_resched() check right before the while (1)?

>> +static void bio_queue_completion(struct bio *bio)
>> +{
>> +	struct bio_complete_batch *batch;
>> +	unsigned long flags;
>> +
>> +	local_lock_irqsave(&bio_complete_batch.lock, flags);
>> +	batch = this_cpu_ptr(&bio_complete_batch);
>> +	bio_list_add(&batch->list, bio);
>> +	local_unlock_irqrestore(&bio_complete_batch.lock, flags);
>> +
>> +	schedule_work_on(smp_processor_id(), &batch->work);
>> +}
> 
> Maybe do something ala:
> 
> static void bio_queue_completion(struct bio *bio)
> {
> 	struct bio_complete_batch *batch;
> 	unsigned long flags;
> 	bool was_empty;
> 
> 	local_lock_irqsave(&bio_complete_batch.lock, flags);
> 	batch = this_cpu_ptr(&bio_complete_batch);
> 	was_empty = bio_list_empty(&batch->list);
> 	bio_list_add(&batch->list, bio);
> 	local_unlock_irqrestore(&bio_complete_batch.lock, flags);
> 
> 	if (was_empty)
> 		schedule_work_on(smp_processor_id(), &batch->work);
> }

Makes sense, will do!

> Outside of these mostly nits, I like this approach. It avoids my main
> worry with this, which was contention on the list locks. And on the
> io_uring side, we'll never hit the !in_task() path anyway, as the
> completions are run from the task always. The bio flag makes sense for
> this.

Thanks! I'm going to give Dave's llist suggestion a shot on top of this as
it seems like it'll simplify this nicely. Looks like that'll involve turning
bio::bi_next into a union with a struct llist_node.

- Tal