Re: [PATCH v3] blk-throtl: Introduce sync and async queues for blk-throtl

[Tejun Heo]

Hello,

On Mon, Dec 26, 2022 at 09:05:05PM +0800, Jinke Han wrote:
>  static void throtl_pending_timer_fn(struct timer_list *t);
> +static inline struct bio *throtl_qnode_bio_list_pop(struct throtl_qnode *qn);

Just define it before the first usage? Also, I think it'd be fine to let the
compiler decide whether to inline.

> +#define BLK_THROTL_SYNC(bio) (bio->bi_opf & (REQ_SYNC | REQ_META | REQ_PRIO))

Nitpick but the above is used only in one place. Does it help to define it
as a macro?

> +/**
> + * throtl_qnode_bio_peek - peek a bio for a qn
> + * @qn: the qnode to peek from
> + *
> + * For read qn, just peek bio from the SYNC queue and return.
> + * For write qn, we first ask the next_to_disp for bio and will pop a bio
> + * to fill it if it's NULL. The next_to_disp is used to pin the bio for
> + * next to dispatch. It is necessary. In the dispatching  process, a peeked
> + * bio may can't be dispatched due to lack of budget and has to wait, the
> + * dispatching process may give up and the spin lock of the request queue
> + * will be released. New bio may be queued in as the spin lock were released.
> + * When it's time to dispatch the waiting bio, another bio may be selected to
> + * check the limit and may be dispatched. If the dispatched bio is smaller
> + * than the waiting bio, the bandwidth may be hard to satisfied as we may
> + * trim the slice after each dispatch.
> + * So pinning the next_to_disp to make sure that the waiting bio and the
> + * dispatched one later always the same one in case that the spin lock of
> + * queue was released and re-holded.

Can you please format it better and proof-read it. I can mostly understand
what it's saying but it can be improved quite a bit. Can you elaborate the
starvation scenario further? What about the [a]sync queue split makes this
more likely?

> +/**
> + * throtl_qnode_bio_pop: pop a bio from sync/async queue
> + * @qn: the qnode to pop a bio from
> + *
> + * For write io qn, the target queue to pop was determined by the disp_sync_cnt.
> + * Try to pop bio from target queue, fetch the bio and return it when it is not
> + * empty. If the target queue empty, pop bio from another queue instead.

How about:

        For reads, always pop from the ASYNC queue. For writes, target SYNC
        or ASYNC queue based on disp_sync_cnt. If empty, try the other
        queue.

> +static inline struct bio *throtl_qnode_bio_list_pop(struct throtl_qnode *qn)
> +{
> +	struct bio *bio;
> +	int from = SYNC;
> +
> +	if (qn->disp_sync_cnt == THROTL_SYNC_FACTOR)
> +		from = ASYNC;

?: often is less readable but I wonder whether it'd be more readable here:

        from = qn->disp_sync_cnt == THROTL_SYNC_FACTOR ? ASYNC : SYNC;

> +
> +	bio = bio_list_pop(&qn->bios[from]);
> +	if (!bio) {
> +		from = 1 - from;
> +		bio = bio_list_pop(&qn->bios[from]);
> +	}
> +
> +	if ((qn->disp_sync_cnt < THROTL_SYNC_FACTOR) &&
> +		(from == SYNC))

Why the line break? Also, this may be more personal preference but I'm not
sure the parentheses are helping much here.

> +		qn->disp_sync_cnt++;
> +	else
> +		qn->disp_sync_cnt = 0;
> +
> +	return bio;
> +}

Thanks.

-- 
tejun