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

Hannes Reinecke
2019-11-26T11:27:54Z

On 11/26/19 12:05 PM, Ming Lei wrote:
> On Tue, Nov 26, 2019 at 10:14:12AM +0100, Hannes Reinecke wrote:
[ .. ]
>> 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().
>
We are allocating tags, I don't think we're even able to modify it at
this point.

> 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.
>
Hmm. Now you get me confused.
In an earlier mail you said:

> This kind of sharing is wrong, sched tags should be request
> queue wide instead of tagset wide, and each request queue has
> its own & independent scheduler queue.

as in v2 we _had_ shared scheduler tags, too.
Did I misread your comment above?

>> if (!hctx->sched_tags)
>> return -ENOMEM;
>>
[ .. ]
>> @@ -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()?
>
Hmm. Indeed, that bit is missing; will be adding it.

>> }
[ .. ]
>> @@ -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.
>
Yes, we need to add a marker here if these are 'normal' or 'scheduler'
tags. This will also help when allocating as then we won't need this
flag twiddling you've complained about.

>> 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/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.
>
Left-overs from merging. Will be removed.

Cheers,

Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@suse.de +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer