Re: [PATCH] blk-throtl: optimize IOPS throttle for large IO scenarios

Tejun Heo
2021-07-16T16:09:15Z

Hello,

On Fri, Jul 16, 2021 at 02:22:49PM +0800, brookxu wrote:
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index a11b3b5..86ff943 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -348,6 +348,8 @@ void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
> trace_block_split(split, (*bio)->bi_iter.bi_sector);
> submit_bio_noacct(*bio);
> *bio = split;
> +
> + blk_throtl_recharge_bio(*bio);

I don't think we're holding the queue lock here.

> }
> }
>
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index b1b22d8..1967438 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -2176,6 +2176,40 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
> }
> #endif
>
> +void blk_throtl_recharge_bio(struct bio *bio)
> +{
> + bool rw = bio_data_dir(bio);
> + struct blkcg_gq *blkg = bio->bi_blkg;
> + struct throtl_grp *tg = blkg_to_tg(blkg);
> + u32 iops_limit = tg_iops_limit(tg, rw);
> +
> + if (iops_limit == UINT_MAX)
> + return;
> +
> + /*
> + * If previous slice expired, start a new one otherwise renew/extend
> + * existing slice to make sure it is at least throtl_slice interval
> + * long since now. New slice is started only for empty throttle group.
> + * If there is queued bio, that means there should be an active
> + * slice and it should be extended instead.
> + */
> + if (throtl_slice_used(tg, rw) && !(tg->service_queue.nr_queued[rw]))
> + throtl_start_new_slice(tg, rw);
> + else {
> + if (time_before(tg->slice_end[rw],
> + jiffies + tg->td->throtl_slice))
> + throtl_extend_slice(tg, rw,
> + jiffies + tg->td->throtl_slice);
> + }
> +
> + /* Recharge the bio to the group, as some BIOs will be further split
> + * after passing through the throttle, causing the actual IOPS to
> + * be greater than the expected value.
> + */
> + tg->last_io_disp[rw]++;
> + tg->io_disp[rw]++;
> +}

But blk-throtl expects queue lock to be held.

How about doing something simpler? Just estimate how many bios a given bio
is gonna be and charge it outright? The calculation will be duplicated
between the split path but that seems like the path of least resistance
here.

Thanks.

--
tejun