Hi Bart,
Just as mentioned in original patch, "dd->async_depth = max(1UL, 3 * q->nr_requests / 4);", this limitation methods look likes won't have a limit effect, because tag allocated is based on sbitmap, not based the whole nr_requests. Right? Thanks!
For write requests, when we assign a tags from sched_tags, data->shallow_depth will be passed to sbitmap_find_bit, see the following code:
nr = sbitmap_find_bit_in_word(&sb->map[index], min_t (unsigned int, __map_depth(sb, index), depth), alloc_hint, wrap);
The smaller of data->shallow_depth and __map_depth(sb, index) will be used as the maximum range when allocating bits.
For a mmc device (one hw queue, deadline I/O scheduler): q->nr_requests = sched_tags = 128, so according to the previous calculation method, dd->async_depth = data->shallow_depth = 96, and the platform is 64bits with 8 cpus, sched_tags.bitmap_tags.sb.shift=5, sb.maps[]=32/32/32/32, 32 is smaller than 96, whether it is a read or a write I/O, tags can be allocated to the maximum range each time, which has not throttling effect.
-----邮件原件----- 发件人: Bart Van Assche bvanassche@acm.org 发送时间: 2024年3月14日 5:42 收件人: Jens Axboe axboe@kernel.dk 抄送: linux-block@vger.kernel.org; Christoph Hellwig hch@lst.de; Bart Van Assche bvanassche@acm.org; stable@vger.kernel.org; Damien Le Moal dlemoal@kernel.org; Harshit Mogalapalli harshit.m.mogalapalli@oracle.com; 牛志国 (Zhiguo Niu) Zhiguo.Niu@unisoc.com 主题: [PATCH] Revert "block/mq-deadline: use correct way to throttling write requests"
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The code "max(1U, 3 * (1U << shift) / 4)" comes from the Kyber I/O scheduler. The Kyber I/O scheduler maintains one internal queue per hwq and hence derives its async_depth from the number of hwq tags. Using this approach for the mq-deadline scheduler is wrong since the mq-deadline scheduler maintains one internal queue for all hwqs combined. Hence this revert.
Cc: stable@vger.kernel.org Cc: Damien Le Moal dlemoal@kernel.org Cc: Harshit Mogalapalli harshit.m.mogalapalli@oracle.com Cc: Zhiguo Niu Zhiguo.Niu@unisoc.com Fixes: d47f9717e5cf ("block/mq-deadline: use correct way to throttling write requests") Signed-off-by: Bart Van Assche bvanassche@acm.org --- block/mq-deadline.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/block/mq-deadline.c b/block/mq-deadline.c index f958e79277b8..02a916ba62ee 100644 --- a/block/mq-deadline.c +++ b/block/mq-deadline.c @@ -646,9 +646,8 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx) struct request_queue *q = hctx->queue; struct deadline_data *dd = q->elevator->elevator_data; struct blk_mq_tags *tags = hctx->sched_tags; - unsigned int shift = tags->bitmap_tags.sb.shift;
- dd->async_depth = max(1U, 3 * (1U << shift) / 4); + dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth); }