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block/mq-deadline: Fix the tag reservation code

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The current tag reservation code is based on a misunderstanding of the
meaning of data->shallow_depth. Fix the tag reservation code as follows:
* By default, do not reserve any tags for synchronous requests because
  for certain use cases reserving tags reduces performance. See also
  Harshit Mogalapalli, [bug-report] Performance regression with fio
  sequential-write on a multipath setup, 2024-03-07
  (https://lore.kernel.org/linux-block/5ce2ae5d-61e2-4ede-ad55-551112602401@oracle.com/)
* Reduce min_shallow_depth to one because min_shallow_depth must be less
  than or equal any shallow_depth value.
* Scale dd->async_depth from the range [1, nr_requests] to [1,
  bits_per_sbitmap_word].

Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <dlemoal@kernel.org>
Cc: Zhiguo Niu <zhiguo.niu@unisoc.com>
Fixes: 07757588e5 ("block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240509170149.7639-3-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Bart Van Assche 2024-05-09 10:01:49 -07:00 committed by Jens Axboe
parent 6259151c04
commit 39823b47bb
1 changed files with 17 additions and 3 deletions
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20
block/mq-deadline.c
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@ -487,6 +487,20 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
return rq;
}
/*
* 'depth' is a number in the range 1..INT_MAX representing a number of
* requests. Scale it with a factor (1 << bt->sb.shift) / q->nr_requests since
* 1..(1 << bt->sb.shift) is the range expected by sbitmap_get_shallow().
* Values larger than q->nr_requests have the same effect as q->nr_requests.
*/
static int dd_to_word_depth(struct blk_mq_hw_ctx *hctx, unsigned int qdepth)
{
struct sbitmap_queue *bt = &hctx->sched_tags->bitmap_tags;
const unsigned int nrr = hctx->queue->nr_requests;
return ((qdepth << bt->sb.shift) + nrr - 1) / nrr;
}
/*
* Called by __blk_mq_alloc_request(). The shallow_depth value set by this
* function is used by __blk_mq_get_tag().
@ -503,7 +517,7 @@ static void dd_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
* Throttle asynchronous requests and writes such that these requests
* do not block the allocation of synchronous requests.
*/
data->shallow_depth = dd->async_depth;
data->shallow_depth = dd_to_word_depth(data->hctx, dd->async_depth);
}
/* Called by blk_mq_update_nr_requests(). */
@ -513,9 +527,9 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
struct deadline_data *dd = q->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;
dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
dd->async_depth = q->nr_requests;
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth);
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, 1);
}
/* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */