Re: [PATCH RFC v6 04/10] blk-mq: Facilitate a shared sbitmap per tagset

[Hannes Reinecke <hare@suse.de>]

On 3/5/20 12:54 PM, John Garry wrote:
> 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>
> ---
>   block/blk-mq-tag.c     | 36 ++++++++++++++++++++++++++++++++++--
>   block/blk-mq-tag.h     | 10 +++++++++-
>   block/blk-mq.c         | 28 ++++++++++++++++++++++++++--
>   block/blk-mq.h         |  5 +++++
>   include/linux/blk-mq.h |  3 +++
>   5 files changed, 77 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index b153ecf8c5c6..dd704118fe83 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -220,7 +220,7 @@ 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;
>   }
> @@ -444,6 +444,7 @@ static int blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
>   		     round_robin, node))
>   		goto free_bitmap_tags;
>   
> +	/* We later overwrite these in case of per-set shared sbitmap */
>   	tags->bitmap_tags = &tags->__bitmap_tags;
>   	tags->breserved_tags = &tags->__breserved_tags;
>   
> @@ -453,7 +454,32 @@ 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->__bitmap_tags, depth, round_robin, node))
> +		return false;
> +	if (bt_alloc(&tag_set->__breserved_tags, tag_set->reserved_tags,
> +		     round_robin, node))
> +		goto free_bitmap_tags;
> +	return true;
> +free_bitmap_tags:
> +	sbitmap_queue_free(&tag_set->__bitmap_tags);
> +	return false;
> +}
> +
> +void blk_mq_exit_shared_sbitmap(struct blk_mq_tag_set *tag_set)
> +{
> +	sbitmap_queue_free(&tag_set->__bitmap_tags);
> +	sbitmap_queue_free(&tag_set->__breserved_tags);
> +}
> +
> +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)
>   {
> @@ -480,6 +506,7 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
>   
>   void blk_mq_free_tags(struct blk_mq_tags *tags)
>   {
> +	/* Do not use tags->{bitmap, breserved}_tags */
>   	sbitmap_queue_free(&tags->__bitmap_tags);
>   	sbitmap_queue_free(&tags->__breserved_tags);
>   	kfree(tags);
Hmm. This is essentially the same as blk_mq_exit_shared_sbitmap().
Please call into that function or clarify the relationship between both.
And modify the comment; it's not about the 'use' but rather the freeing 
of the structures, and ->bitmap/breserved just point to the __ variants.

> @@ -538,6 +565,11 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>   	return 0;
>   }
>   
> +void blk_mq_tag_resize_shared_sbitmap(struct blk_mq_tag_set *set, unsigned int size)
> +{
> +	sbitmap_queue_resize(&set->__bitmap_tags, size - set->reserved_tags);
> +}
> +
>   /**
>    * blk_mq_unique_tag() - return a tag that is unique queue-wide
>    * @rq: request for which to compute a unique tag
> diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> index 4f866a39b926..0a57e4f041a9 100644
> --- a/block/blk-mq-tag.h
> +++ b/block/blk-mq-tag.h
> @@ -25,7 +25,12 @@ 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 void blk_mq_exit_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);
> @@ -34,6 +39,9 @@ extern void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
>   extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>   					struct blk_mq_tags **tags,
>   					unsigned int depth, bool can_grow);
> +extern void blk_mq_tag_resize_shared_sbitmap(struct blk_mq_tag_set *set,
> +					     unsigned int size);
> +
>   extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
>   void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
>   		void *priv);
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 375f5064c183..d10e24bee7d9 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2126,7 +2126,7 @@ 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;
> @@ -2980,8 +2980,10 @@ static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
>   	return 0;
>   
>   out_unwind:
> -	while (--i >= 0)
> +	while (--i >= 0) {
>   		blk_mq_free_rq_map(set->tags[i]);
> +		set->tags[i] = NULL;
> +	}
>   
>   	return -ENOMEM;
>   }
> @@ -3147,11 +3149,28 @@ 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_rq_maps;
> +		}
> +
> +		for (i = 0; i < set->nr_hw_queues; i++) {
> +			struct blk_mq_tags *tags = set->tags[i];
> +
> +			tags->bitmap_tags = &set->__bitmap_tags;
> +			tags->breserved_tags = &set->__breserved_tags;
> +		}
> +	}
> +
>   	mutex_init(&set->tag_list_lock);
>   	INIT_LIST_HEAD(&set->tag_list);
>   
>   	return 0;
>   
> +out_free_mq_rq_maps:
> +	for (i = 0; i < set->nr_hw_queues; i++)
> +		blk_mq_free_rq_map(set->tags[i]);
>   out_free_mq_map:
>   	for (i = 0; i < set->nr_maps; i++) {
>   		kfree(set->map[i].mq_map);
> @@ -3170,6 +3189,9 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
>   	for (i = 0; i < set->nr_hw_queues; i++)
>   		blk_mq_free_map_and_requests(set, i);
>   
> +	if (blk_mq_is_sbitmap_shared(set))
> +		blk_mq_exit_shared_sbitmap(set);
> +
>   	for (j = 0; j < set->nr_maps; j++) {
>   		kfree(set->map[j].mq_map);
>   		set->map[j].mq_map = NULL;
> @@ -3206,6 +3228,8 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>   		if (!hctx->sched_tags) {
>   			ret = blk_mq_tag_update_depth(hctx, &hctx->tags, nr,
>   							false);
> +			if (!ret && blk_mq_is_sbitmap_shared(set))
> +				blk_mq_tag_resize_shared_sbitmap(set, nr);
>   		} else {
>   			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
>   							nr, true);
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 10bfdfb494fa..dde2d29f0ce5 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -158,6 +158,11 @@ 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;
> +}
> +
When is that set?
And to my understanding it contingent on the ->bitmap pointer _not_ 
pointing to ->__bitmap.
Can we use this as a test and drop this flag?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
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