Hi Hannes,
Thanks for taking a crack at the issue. My comments below..
On Tue, 4 Apr 2017, 5:07am, Hannes Reinecke wrote:
> Most legacy HBAs have a tagset per HBA, not per queue. To map
> these devices onto block-mq this patch implements a new tagset
> flag BLK_MQ_F_GLOBAL_TAGS, which will cause the tag allocator
> to use just one tagset for all hardware queues.
>
> Signed-off-by: Hannes Reinecke <h...@suse.com>
> ---
> block/blk-mq-tag.c | 12 ++++++++----
> block/blk-mq.c | 10 ++++++++--
> include/linux/blk-mq.h | 1 +
> 3 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index e48bc2c..a14e76c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -276,9 +276,11 @@ static void blk_mq_all_tag_busy_iter(struct blk_mq_tags
> *tags,
> void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> busy_tag_iter_fn *fn, void *priv)
> {
> - int i;
> + int i, lim = tagset->nr_hw_queues;
>
> - for (i = 0; i < tagset->nr_hw_queues; i++) {
> + if (tagset->flags & BLK_MQ_F_GLOBAL_TAGS)
> + lim = 1;
> + for (i = 0; i < lim; i++) {
> if (tagset->tags && tagset->tags[i])
> blk_mq_all_tag_busy_iter(tagset->tags[i], fn, priv);
> }
> @@ -287,12 +289,14 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set
> *tagset,
>
> int blk_mq_reinit_tagset(struct blk_mq_tag_set *set)
> {
> - int i, j, ret = 0;
> + int i, j, ret = 0, lim = set->nr_hw_queues;
>
> if (!set->ops->reinit_request)
> goto out;
>
> - for (i = 0; i < set->nr_hw_queues; i++) {
> + if (set->flags & BLK_MQ_F_GLOBAL_TAGS)
> + lim = 1;
> + for (i = 0; i < lim; i++) {
> struct blk_mq_tags *tags = set->tags[i];
>
> for (j = 0; j < tags->nr_tags; j++) {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 159187a..db96ed0 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2061,6 +2061,10 @@ static bool __blk_mq_alloc_rq_map(struct
> blk_mq_tag_set *set, int hctx_idx)
> {
> int ret = 0;
>
> + if ((set->flags & BLK_MQ_F_GLOBAL_TAGS) && hctx_idx != 0) {
> + set->tags[hctx_idx] = set->tags[0];
> + return true;
> + }
So, this effectively make all request allocations to the same NUMA node
locality of the hctx_idx 0, correct? Is the performance hit you were
talking about in the cover letter?
Do you have any other alternatives in mind? Dynamic growing/shrinking
tags/request-pool in hctx with a fixed base as start?
One alternative that comes to my mind is to move the divvy up logic to
SCSI (instead of LLD doing it), IOW:
1. Have SCSI set tag_set.queue_depth to can_queue/nr_hw_queues
2. Have blk_mq_unique_tag() (or a new i/f) returning "hwq * nr_hw_queue +
rq->tag"
That would make the tags linear in the can_queue space, but could result
in poor use of LLD resource if a given hctx has used up all it's tags.
On a related note, would not the current use of can_queue in SCSI lead to
poor resource utilization in MQ cases? Like, block layer allocating
nr_hw_queues * tags+request+driver_data.etc * can_queue, but SCSI limiting
the number of requests to can_queue.
BTW, if you would like me to try out this patch on my setup, please let me
know.
Regards,
-Arun
> set->tags[hctx_idx] = blk_mq_alloc_rq_map(set, hctx_idx,
> set->queue_depth, set->reserved_tags);
> if (!set->tags[hctx_idx])
> @@ -2080,8 +2084,10 @@ static void blk_mq_free_map_and_requests(struct
> blk_mq_tag_set *set,
> unsigned int hctx_idx)
> {
> if (set->tags[hctx_idx]) {
> - blk_mq_free_rqs(set, set->tags[hctx_idx], hctx_idx);
> - blk_mq_free_rq_map(set->tags[hctx_idx]);
> + if (!(set->flags & BLK_MQ_F_GLOBAL_TAGS) || hctx_idx == 0) {
> + blk_mq_free_rqs(set, set->tags[hctx_idx], hctx_idx);
> + blk_mq_free_rq_map(set->tags[hctx_idx]);
> + }
> set->tags[hctx_idx] = NULL;
> }
> }
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index b296a90..eee27b016 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -155,6 +155,7 @@ enum {
> BLK_MQ_F_DEFER_ISSUE = 1 << 4,
> BLK_MQ_F_BLOCKING = 1 << 5,
> BLK_MQ_F_NO_SCHED = 1 << 6,
> + BLK_MQ_F_GLOBAL_TAGS = 1 << 7,
> BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
> BLK_MQ_F_ALLOC_POLICY_BITS = 1,
>
>
