Tejun Heo wrote on 2021/7/17 0:09:

Hello,

On Fri, Jul 16, 2021 at 02:22:49PM +0800, brookxu wrote:

quoted

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.

sorry, some kind of synchronization mechanism is really needed here. But the use of queue_lock
here may be unsafe, since it is difficult for us to control the lock on the split path.

quoted

 	}
 }
 

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.

I have tried this method, the code redundancy is indeed a bit high, it may not be
very convenient for code maintenance. In addition to this problem, since we add
a large value at a time, the fluctuation of IOPS will be relatively large. Since
blk_throtl_recharge_bio() does not need to participate in the maintenance of the
state machine, we only need to protect some fields of tg, so can we add a new
spin_lock to tg instead of queue_lock to solve the synchronization problem ? Just
a idea, Thanks.

Thanks.