Re: [PATCH 4/8] blk-mq: Facilitate a shared sbitmap per tagset

Ming Lei
2019-11-26T11:05:50Z

On Tue, Nov 26, 2019 at 10:14:12AM +0100, Hannes Reinecke wrote:
> From: John Garry <john.garry@huawei.com>
>
> Some SCSI HBAs (such as HPSA, megaraid, mpt3sas, hisi_sas_v3 ..) support
> multiple reply queues with single hostwide tags.
>
> In addition, these drivers want to use interrupt assignment in
> pci_alloc_irq_vectors(PCI_IRQ_AFFINITY). However, as discussed in [0],
> CPU hotplug may cause in-flight IO completion to not be serviced when an
> interrupt is shutdown.
>
> To solve that problem, Ming's patchset to drain hctx's should ensure no
> IOs are missed in-flight [1].
>
> However, to take advantage of that patchset, we need to map the HBA HW
> queues to blk mq hctx's; to do that, we need to expose the HBA HW queues.
>
> In making that transition, the per-SCSI command request tags are no
> longer unique per Scsi host - they are just unique per hctx. As such, the
> HBA LLDD would have to generate this tag internally, which has a certain
> performance overhead.
>
> However another problem is that blk mq assumes the host may accept
> (Scsi_host.can_queue * #hw queue) commands. In [2], we removed the Scsi
> host busy counter, which would stop the LLDD being sent more than
> .can_queue commands; however, we should still ensure that the block layer
> does not issue more than .can_queue commands to the Scsi host.
>
> To solve this problem, introduce a shared sbitmap per blk_mq_tag_set,
> which may be requested at init time.
>
> New flag BLK_MQ_F_TAG_HCTX_SHARED should be set when requesting the
> tagset to indicate whether the shared sbitmap should be used.
>
> Even when BLK_MQ_F_TAG_HCTX_SHARED is set, we still allocate a full set of
> tags and requests per hctx; the reason for this is that if we only allocate
> tags and requests for a single hctx - like hctx0 - we may break block
> drivers which expect a request be associated with a specific hctx, i.e.
> not hctx0.
>
> This is based on work originally from Ming Lei in [3] and from Bart's
> suggestion in [4].
>
> [0] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/
> [1] https://lore.kernel.org/linux-block/20191014015043.25029-1-ming.lei@redhat.com/
> [2] https://lore.kernel.org/linux-scsi/20191025065855.6309-1-ming.lei@redhat.com/
> [3] https://lore.kernel.org/linux-block/20190531022801.10003-1-ming.lei@redhat.com/
> [4] https://lore.kernel.org/linux-block/ff77beff-5fd9-9f05-12b6-826922bace1f@huawei.com/T/#m3db0a602f095cbcbff27e9c884d6b4ae826144be
>
> Signed-off-by: John Garry <john.garry@huawei.com>
> Signed-off-by: Hannes Reinecke <hare@suse.de>
> ---
> block/blk-mq-sched.c | 8 ++++++--
> block/blk-mq-tag.c | 36 +++++++++++++++++++++++++++++-------
> block/blk-mq-tag.h | 6 +++++-
> block/blk-mq.c | 45 ++++++++++++++++++++++++++++++++++++---------
> block/blk-mq.h | 7 ++++++-
> include/linux/blk-mq.h | 7 +++++++
> 6 files changed, 89 insertions(+), 20 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index ca22afd47b3d..f3589f42b96d 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -452,7 +452,7 @@ static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
> {
> if (hctx->sched_tags) {
> blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
> - blk_mq_free_rq_map(hctx->sched_tags);
> + blk_mq_free_rq_map(hctx->sched_tags, false);
> hctx->sched_tags = NULL;
> }
> }
> @@ -462,10 +462,14 @@ static int blk_mq_sched_alloc_tags(struct request_queue *q,
> unsigned int hctx_idx)
> {
> struct blk_mq_tag_set *set = q->tag_set;
> + int flags = set->flags;
> int ret;
>
> + /* Scheduler tags are never shared */
> + set->flags &= ~BLK_MQ_F_TAG_HCTX_SHARED;
> hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
> set->reserved_tags);
> + set->flags = flags;

This way is very fragile, race is made against other uses of
blk_mq_is_sbitmap_shared().

From performance viewpoint, all hctx belonging to this request queue should
share one scheduler tagset in case of BLK_MQ_F_TAG_HCTX_SHARED, cause
driver tag queue depth isn't changed.

> if (!hctx->sched_tags)
> return -ENOMEM;
>
> @@ -484,7 +488,7 @@ static void blk_mq_sched_tags_teardown(struct request_queue *q)
>
> queue_for_each_hw_ctx(q, hctx, i) {
> if (hctx->sched_tags) {
> - blk_mq_free_rq_map(hctx->sched_tags);
> + blk_mq_free_rq_map(hctx->sched_tags, false);
> hctx->sched_tags = NULL;
> }
> }
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 332d71cd3976..53b4c4c53c6a 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -228,7 +228,7 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> * We can hit rq == NULL here, because the tagging functions
> * test and set the bit before assigning ->rqs[].
> */
> - if (rq && rq->q == hctx->queue)
> + if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx)
> return iter_data->fn(hctx, rq, iter_data->data, reserved);
> return true;
> }
> @@ -470,7 +470,27 @@ static int blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
> return -ENOMEM;
> }
>
> -struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
> +bool blk_mq_init_shared_sbitmap(struct blk_mq_tag_set *tag_set)
> +{
> + unsigned int depth = tag_set->queue_depth -tag_set->reserved_tags;
> + int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(tag_set->flags);
> + bool round_robin = alloc_policy == BLK_TAG_ALLOC_RR;
> + int node = tag_set->numa_node;
> +
> + if (bt_alloc(&tag_set->shared_bitmap_tags, depth, round_robin, node))
> + return false;
> + if (bt_alloc(&tag_set->shared_breserved_tags, tag_set->reserved_tags, round_robin,
> + node))
> + goto free_bitmap_tags;
> +
> + return true;
> +free_bitmap_tags:
> + sbitmap_queue_free(&tag_set->shared_bitmap_tags);
> + return false;
> +}
> +
> +struct blk_mq_tags *blk_mq_init_tags(struct blk_mq_tag_set *set,
> + unsigned int total_tags,
> unsigned int reserved_tags,
> int node, int alloc_policy)
> {
> @@ -488,9 +508,11 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
> tags->nr_tags = total_tags;
> tags->nr_reserved_tags = reserved_tags;
>
> - if (blk_mq_init_bitmap_tags(tags, node, alloc_policy) < 0) {
> - kfree(tags);
> - tags = NULL;
> + if (!blk_mq_is_sbitmap_shared(set)) {
> + if (blk_mq_init_bitmap_tags(tags, node, alloc_policy) < 0) {
> + kfree(tags);
> + tags = NULL;
> + }
> }
> return tags;
> }
> @@ -538,12 +560,12 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
> return -ENOMEM;
> ret = blk_mq_alloc_rqs(set, new, hctx->queue_num, tdepth);
> if (ret) {
> - blk_mq_free_rq_map(new);
> + blk_mq_free_rq_map(new, blk_mq_is_sbitmap_shared(set));
> return -ENOMEM;
> }
>
> blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
> - blk_mq_free_rq_map(*tagsptr);
> + blk_mq_free_rq_map(*tagsptr, blk_mq_is_sbitmap_shared(set));
> *tagsptr = new;
> } else {
> /*
> diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> index 10b66fd4664a..279a861c7e58 100644
> --- a/block/blk-mq-tag.h
> +++ b/block/blk-mq-tag.h
> @@ -22,7 +22,11 @@ struct blk_mq_tags {
> };
>
>
> -extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags, unsigned int reserved_tags, int node, int alloc_policy);
> +extern bool blk_mq_init_shared_sbitmap(struct blk_mq_tag_set *tag_set);
> +extern struct blk_mq_tags *blk_mq_init_tags(struct blk_mq_tag_set *tag_set,
> + unsigned int nr_tags,
> + unsigned int reserved_tags,
> + int node, int alloc_policy);
> extern void blk_mq_free_tags(struct blk_mq_tags *tags);
>
> extern unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data);
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f0c40fcbd8ae..db87b0f57dbe 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2075,13 +2075,14 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> }
> }
>
> -void blk_mq_free_rq_map(struct blk_mq_tags *tags)
> +void blk_mq_free_rq_map(struct blk_mq_tags *tags, bool shared)
> {
> kfree(tags->rqs);
> tags->rqs = NULL;
> kfree(tags->static_rqs);
> tags->static_rqs = NULL;
> - blk_mq_free_tags(tags);
> + if (!shared)
> + blk_mq_free_tags(tags);
> }
>
> struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> @@ -2096,7 +2097,7 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> if (node == NUMA_NO_NODE)
> node = set->numa_node;
>
> - tags = blk_mq_init_tags(nr_tags, reserved_tags, node,
> + tags = blk_mq_init_tags(set, nr_tags, reserved_tags, node,
> BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags));
> if (!tags)
> return NULL;
> @@ -2105,7 +2106,8 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY,
> node);
> if (!tags->rqs) {
> - blk_mq_free_tags(tags);
> + if (!blk_mq_is_sbitmap_shared(set))
> + blk_mq_free_tags(tags);
> return NULL;
> }
>
> @@ -2114,7 +2116,8 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> node);
> if (!tags->static_rqs) {
> kfree(tags->rqs);
> - blk_mq_free_tags(tags);
> + if (!blk_mq_is_sbitmap_shared(set))
> + blk_mq_free_tags(tags);
> return NULL;
> }
>
> @@ -2446,7 +2449,7 @@ static bool __blk_mq_alloc_rq_map(struct blk_mq_tag_set *set, int hctx_idx)
> if (!ret)
> return true;
>
> - blk_mq_free_rq_map(set->tags[hctx_idx]);
> + blk_mq_free_rq_map(set->tags[hctx_idx], blk_mq_is_sbitmap_shared(set));
> set->tags[hctx_idx] = NULL;
> return false;
> }
> @@ -2456,7 +2459,8 @@ static void blk_mq_free_map_and_requests(struct blk_mq_tag_set *set,
> {
> if (set->tags && set->tags[hctx_idx]) {
> blk_mq_free_rqs(set, set->tags[hctx_idx], hctx_idx);
> - blk_mq_free_rq_map(set->tags[hctx_idx]);
> + blk_mq_free_rq_map(set->tags[hctx_idx],
> + blk_mq_is_sbitmap_shared(set));
> set->tags[hctx_idx] = NULL;
> }

Who will free the shared tags finally in case of blk_mq_is_sbitmap_shared()?

> }
> @@ -2954,7 +2958,7 @@ static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
>
> out_unwind:
> while (--i >= 0)
> - blk_mq_free_rq_map(set->tags[i]);
> + blk_mq_free_rq_map(set->tags[i], blk_mq_is_sbitmap_shared(set));
>
> return -ENOMEM;
> }
> @@ -3099,6 +3103,20 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
> if (ret)
> goto out_free_mq_map;
>
> + if (blk_mq_is_sbitmap_shared(set)) {
> + if (!blk_mq_init_shared_sbitmap(set)) {
> + ret = -ENOMEM;
> + goto out_free_mq_map;
> + }
> +
> + for (i = 0; i < set->nr_hw_queues; i++) {
> + struct blk_mq_tags *tags = set->tags[i];
> +
> + tags->bitmap_tags = &set->shared_bitmap_tags;
> + tags->breserved_tags = &set->shared_breserved_tags;
> + }
> + }
> +
> mutex_init(&set->tag_list_lock);
> INIT_LIST_HEAD(&set->tag_list);
>
> @@ -3168,8 +3186,17 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
> q->elevator->type->ops.depth_updated(hctx);
> }
>
> - if (!ret)
> + if (!ret) {
> + if (blk_mq_is_sbitmap_shared(set)) {
> + sbitmap_queue_resize(&set->shared_bitmap_tags, nr);
> + sbitmap_queue_resize(&set->shared_breserved_tags, nr);
> + }

The above change is wrong in case of hctx->sched_tags.

> q->nr_requests = nr;
> + }
> + /*
> + * if ret != 0, q->nr_requests would not be updated, yet the depth
> + * for some hctx may have changed - is that right?
> + */
>
> blk_mq_unquiesce_queue(q);
> blk_mq_unfreeze_queue(q);
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 78d38b5f2793..c4b8213dfdfc 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -53,7 +53,7 @@ struct request *blk_mq_dequeue_from_ctx(struct blk_mq_hw_ctx *hctx,
> */
> void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> unsigned int hctx_idx);
> -void blk_mq_free_rq_map(struct blk_mq_tags *tags);
> +void blk_mq_free_rq_map(struct blk_mq_tags *tags, bool shared);
> struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> unsigned int hctx_idx,
> unsigned int nr_tags,
> @@ -166,6 +166,11 @@ struct blk_mq_alloc_data {
> struct blk_mq_hw_ctx *hctx;
> };
>
> +static inline bool blk_mq_is_sbitmap_shared(struct blk_mq_tag_set *tag_set)
> +{
> + return !!(tag_set->flags & BLK_MQ_F_TAG_HCTX_SHARED);
> +}
> +
> static inline struct blk_mq_tags *blk_mq_tags_from_data(struct blk_mq_alloc_data *data)
> {
> if (data->flags & BLK_MQ_REQ_INTERNAL)
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 147185394a25..670e9a949d32 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -109,6 +109,12 @@ struct blk_mq_tag_set {
> unsigned int flags; /* BLK_MQ_F_* */
> void *driver_data;
>
> + struct sbitmap_queue shared_bitmap_tags;
> + struct sbitmap_queue shared_breserved_tags;
> +
> + struct sbitmap_queue sched_shared_bitmap_tags;
> + struct sbitmap_queue sched_shared_breserved_tags;
> +

The above two fields aren't used in this patch.

> struct blk_mq_tags **tags;
>
> struct mutex tag_list_lock;
> @@ -226,6 +232,7 @@ struct blk_mq_ops {
> enum {
> BLK_MQ_F_SHOULD_MERGE = 1 << 0,
> BLK_MQ_F_TAG_QUEUE_SHARED = 1 << 1,
> + BLK_MQ_F_TAG_HCTX_SHARED = 1 << 2,
> BLK_MQ_F_BLOCKING = 1 << 5,
> BLK_MQ_F_NO_SCHED = 1 << 6,
> BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
> --
> 2.16.4
>

--
Ming