linux-kernel - [PATCH 2/7] blk-mq: count changed hctx as active in blk_mq_get

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Date:   Fri, 10 Feb 2023 04:11:11 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     axboe@...nel.dk, hch@....de, jack@...e.cz
Cc:     andriy.shevchenko@...ux.intel.com, qiulaibin@...wei.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/7] blk-mq: count changed hctx as active in blk_mq_get_tag

Commit d263ed9926823 ("blk-mq: count the hctx as active before allocating
tag") active hctx before blk_mq_get_tag to avoid petential starvation.
However, the hctx to alloc tag may change in blk_mq_get_tag if
BLK_MQ_REQ_NOWAIT is not set, then there are two problems:
1. The hctx without real allocation is marked active.
2. The starvation problem mentioned in Commit d263ed9926823 ("blk-mq:
count the hctx as active before allocating tag") still exists on the
changed hctx as it maybe not marked active before tag allocation.

For problem 1, the hctx which is marked active probably gets IO
soon or will be marked inactive after lazy detection of tag idle.
Mark changed hctx active to fix problem 2.

Fixes: d263ed992682 ("blk-mq: count the hctx as active before allocating tag")
Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
---
 block/blk-mq-tag.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 1d3135acfc98..e566fd96dc26 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -191,6 +191,9 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		data->ctx = blk_mq_get_ctx(data->q);
 		data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
 						data->ctx);
+		if (!(data->rq_flags & RQF_ELV))
+			blk_mq_tag_busy(data->hctx);
+
 		tags = blk_mq_tags_from_data(data);
 		if (data->flags & BLK_MQ_REQ_RESERVED)
 			bt = &tags->breserved_tags;
-- 
2.30.0