Re: [PATCH v3 2/2] blk-mq: Use request queue-wide tags for tagset-wide sbitmap

From: Ming Lei
Date: Thu May 13 2021 - 22:05:57 EST

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In reply to: John Garry: "[PATCH v3 2/2] blk-mq: Use request queue-wide tags for tagset-wide sbitmap"
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On Thu, May 13, 2021 at 08:00:58PM +0800, John Garry wrote:
> The tags used for an IO scheduler are currently per hctx.
>
> As such, when q->nr_hw_queues grows, so does the request queue total IO
> scheduler tag depth.
>
> This may cause problems for SCSI MQ HBAs whose total driver depth is
> fixed.
>
> Ming and Yanhui report higher CPU usage and lower throughput in scenarios
> where the fixed total driver tag depth is appreciably lower than the total
> scheduler tag depth:
> https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@xxxxxxxxxx/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b
>
> In that scenario, since the scheduler tag is got first, much contention
> is introduced since a driver tag may not be available after we have got
> the sched tag.
>
> Improve this scenario by introducing request queue-wide tags for when
> a tagset-wide sbitmap is used. The static sched requests are still
> allocated per hctx, as requests are initialised per hctx, as in
> blk_mq_init_request(..., hctx_idx, ...) ->
> set->ops->init_request(.., hctx_idx, ...).
>
> For simplicity of resizing the request queue sbitmap when updating the
> request queue depth, just init at the max possible size, so we don't need
> to deal with the possibly with swapping out a new sbitmap for old if
> we need to grow.
>
> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> ---
> block/blk-mq-sched.c | 67 ++++++++++++++++++++++++++++++++++--------
> block/blk-mq-sched.h | 2 ++
> block/blk-mq-tag.c | 11 ++++---
> block/blk-mq.c | 13 ++++++--
> include/linux/blkdev.h | 4 +++
> 5 files changed, 76 insertions(+), 21 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 42a365b1b9c0..6485969fce9c 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -507,11 +507,9 @@ static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
> struct blk_mq_hw_ctx *hctx,
> unsigned int hctx_idx)
> {
> - unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> -
> if (hctx->sched_tags) {
> blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
> - blk_mq_free_rq_map(hctx->sched_tags, flags);
> + blk_mq_free_rq_map(hctx->sched_tags, set->flags);
> hctx->sched_tags = NULL;
> }
> }
> @@ -521,12 +519,10 @@ static int blk_mq_sched_alloc_tags(struct request_queue *q,
> unsigned int hctx_idx)
> {
> struct blk_mq_tag_set *set = q->tag_set;
> - /* Clear HCTX_SHARED so tags are init'ed */
> - unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> int ret;
>
> hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
> - set->reserved_tags, flags);
> + set->reserved_tags, set->flags);
> if (!hctx->sched_tags)
> return -ENOMEM;
>
> @@ -544,16 +540,50 @@ static void blk_mq_sched_tags_teardown(struct request_queue *q)
> int i;
>
> queue_for_each_hw_ctx(q, hctx, i) {
> - /* Clear HCTX_SHARED so tags are freed */
> - unsigned int flags = hctx->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> -
> if (hctx->sched_tags) {
> - blk_mq_free_rq_map(hctx->sched_tags, flags);
> + blk_mq_free_rq_map(hctx->sched_tags, hctx->flags);
> hctx->sched_tags = NULL;
> }
> }
> }
>
> +static int blk_mq_init_sched_shared_sbitmap(struct request_queue *queue)
> +{
> + struct blk_mq_tag_set *set = queue->tag_set;
> + int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags);
> + struct blk_mq_hw_ctx *hctx;
> + int ret, i;
> +
> + /*
> + * Set initial depth at max so that we don't need to reallocate for
> + * updating nr_requests.
> + */
> + ret = blk_mq_init_bitmaps(&queue->sched_bitmap_tags,
> + &queue->sched_breserved_tags,
> + MAX_SCHED_RQ, set->reserved_tags,
> + set->numa_node, alloc_policy);
> + if (ret)
> + return ret;
> +
> + queue_for_each_hw_ctx(queue, hctx, i) {
> + hctx->sched_tags->bitmap_tags =
> + &queue->sched_bitmap_tags;
> + hctx->sched_tags->breserved_tags =
> + &queue->sched_breserved_tags;
> + }
> +
> + sbitmap_queue_resize(&queue->sched_bitmap_tags,
> + queue->nr_requests - set->reserved_tags);
> +
> + return 0;
> +}
> +
> +static void blk_mq_exit_sched_shared_sbitmap(struct request_queue *queue)
> +{
> + sbitmap_queue_free(&queue->sched_bitmap_tags);
> + sbitmap_queue_free(&queue->sched_breserved_tags);
> +}
> +
> int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> {
> struct blk_mq_hw_ctx *hctx;
> @@ -578,12 +608,18 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> queue_for_each_hw_ctx(q, hctx, i) {
> ret = blk_mq_sched_alloc_tags(q, hctx, i);
> if (ret)
> - goto err;
> + goto err_free_tags;
> + }
> +
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags)) {
> + ret = blk_mq_init_sched_shared_sbitmap(q);
> + if (ret)
> + goto err_free_tags;
> }
>
> ret = e->ops.init_sched(q, e);
> if (ret)
> - goto err;
> + goto err_free_sbitmap;
>
> blk_mq_debugfs_register_sched(q);
>
> @@ -603,7 +639,10 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
>
> return 0;
>
> -err:
> +err_free_sbitmap:
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
> + blk_mq_exit_sched_shared_sbitmap(q);
> +err_free_tags:
> blk_mq_sched_free_requests(q);
> blk_mq_sched_tags_teardown(q);
> q->elevator = NULL;
> @@ -641,5 +680,7 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
> if (e->type->ops.exit_sched)
> e->type->ops.exit_sched(e);
> blk_mq_sched_tags_teardown(q);
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
> + blk_mq_exit_sched_shared_sbitmap(q);
> q->elevator = NULL;
> }
> diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
> index 5b18ab915c65..aff037cfd8e7 100644
> --- a/block/blk-mq-sched.h
> +++ b/block/blk-mq-sched.h
> @@ -5,6 +5,8 @@
> #include "blk-mq.h"
> #include "blk-mq-tag.h"
>
> +#define MAX_SCHED_RQ (16 * BLKDEV_MAX_RQ)
> +
> void blk_mq_sched_assign_ioc(struct request *rq);
>
> bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 45479c0f88a2..c65d1b8891cf 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -13,6 +13,7 @@
> #include <linux/delay.h>
> #include "blk.h"
> #include "blk-mq.h"
> +#include "blk-mq-sched.h"
> #include "blk-mq-tag.h"
>
> /*
> @@ -563,8 +564,6 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
> */
> if (tdepth > tags->nr_tags) {
> struct blk_mq_tag_set *set = hctx->queue->tag_set;
> - /* Only sched tags can grow, so clear HCTX_SHARED flag */
> - unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> struct blk_mq_tags *new;
> bool ret;
>
> @@ -575,21 +574,21 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
> * We need some sort of upper limit, set it high enough that
> * no valid use cases should require more.
> */
> - if (tdepth > 16 * BLKDEV_MAX_RQ)
> + if (tdepth > MAX_SCHED_RQ)
> return -EINVAL;
>
> new = blk_mq_alloc_rq_map(set, hctx->queue_num, tdepth,
> - tags->nr_reserved_tags, flags);
> + tags->nr_reserved_tags, set->flags);
> if (!new)
> return -ENOMEM;
> ret = blk_mq_alloc_rqs(set, new, hctx->queue_num, tdepth);
> if (ret) {
> - blk_mq_free_rq_map(new, flags);
> + blk_mq_free_rq_map(new, set->flags);
> return -ENOMEM;
> }
>
> blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
> - blk_mq_free_rq_map(*tagsptr, flags);
> + blk_mq_free_rq_map(*tagsptr, set->flags);
> *tagsptr = new;
> } else {
> /*
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 499ad5462f7e..8b5ecc801d3f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3564,15 +3564,24 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
> } else {
> ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
> nr, true);
> + if (blk_mq_is_sbitmap_shared(set->flags)) {
> + hctx->sched_tags->bitmap_tags =
> + &q->sched_bitmap_tags;
> + hctx->sched_tags->breserved_tags =
> + &q->sched_breserved_tags;
> + }
> }
> if (ret)
> break;
> if (q->elevator && q->elevator->type->ops.depth_updated)
> q->elevator->type->ops.depth_updated(hctx);
> }
> -
> - if (!ret)
> + if (!ret) {
> q->nr_requests = nr;
> + if (q->elevator && blk_mq_is_sbitmap_shared(set->flags))
> + sbitmap_queue_resize(&q->sched_bitmap_tags,
> + nr - set->reserved_tags);
> + }
>
> blk_mq_unquiesce_queue(q);
> blk_mq_unfreeze_queue(q);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 1255823b2bc0..4092c2a38f10 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -25,6 +25,7 @@
> #include <linux/scatterlist.h>
> #include <linux/blkzoned.h>
> #include <linux/pm.h>
> +#include <linux/sbitmap.h>
>
> struct module;
> struct scsi_ioctl_command;
> @@ -493,6 +494,9 @@ struct request_queue {
>
> atomic_t nr_active_requests_shared_sbitmap;
>
> + struct sbitmap_queue sched_bitmap_tags;
> + struct sbitmap_queue sched_breserved_tags;
> +
> struct list_head icq_list;
> #ifdef CONFIG_BLK_CGROUP
> DECLARE_BITMAP (blkcg_pols, BLKCG_MAX_POLS);
> --
> 2.26.2
>

Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>

Thanks,
Ming

Next message: kernel test robot: "[mm/gup] 31b912de13: phoronix-test-suite.npb.FT.A.total_mop_s 61.8% improvement"
Previous message: Ming Lei: "Re: [PATCH v3 1/2] blk-mq: Some tag allocation code refactoring"
In reply to: John Garry: "[PATCH v3 2/2] blk-mq: Use request queue-wide tags for tagset-wide sbitmap"
Next in thread: John Garry: "[PATCH v3 1/2] blk-mq: Some tag allocation code refactoring"
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