commit 39823b47bbd40502632ffba90ebb34fff7c8b5e8 upstream.

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@ora...) * 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: 07757588e507 ("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 --- block/mq-deadline.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c index f10c2a0d18d4..ff029e1acc4d 100644 --- a/block/mq-deadline.c +++ b/block/mq-deadline.c @@ -597,6 +597,20 @@ 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(). @@ -613,7 +627,7 @@ 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(). */ @@ -623,9 +637,9 @@ 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(). */