Re: [PATCH RFC v7 06/12] blk-mq: Record active_queues_shared_sbitmap per tag_set for when using shared sbitmap

John Garry
2020-06-11T14:24:16Z

>>   out:
>> @@ -802,6 +822,7 @@ static const struct blk_mq_debugfs_attr
>> blk_mq_debugfs_hctx_shared_sbitmap_attrs
>>       {"dispatch", 0400, .seq_ops = &hctx_dispatch_seq_ops},
>>       {"busy", 0400, hctx_busy_show},
>>       {"ctx_map", 0400, hctx_ctx_map_show},
>> +    {"tags", 0400, hctx_tags_show},
>>       {"sched_tags", 0400, hctx_sched_tags_show},
>>       {"sched_tags_bitmap", 0400, hctx_sched_tags_bitmap_show},
>>       {"io_poll", 0600, hctx_io_poll_show, hctx_io_poll_write},
>
> I had been pondering this, too, when creating v6. Problem is that it'll
> show the tags per hctx, but as they are shared I guess the list looks
> pretty identical per hctx.

Right, so my main concern in this series is debugfs, and how to present
the tag/hctx info. We can hide things under the hood mostly, but not so
much for debugfs.

> So I guess we should filter the tags per hctx to have only those active
> on that hctx displayed. But when doing so we can only print the
> in-flight tags, the others are not assigned to a hctx and as such we
> can't make a decision on which hctx they'll end up.

So we filter the tags in 7/12, and that should be ok.

But a concern is that we present identical hctx_tags_show() ->
blk_mq_debugfs_shared_sitmap_show() -> sbitmap_queue_show() per-hctx
info, which may be inappropriate/misleading/wrong. I need to audit that
more thoroughly.

>
>> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
>> index 7db16e49f6f6..6ca06b1c3a99 100644
>> --- a/block/blk-mq-tag.c
>> +++ b/block/blk-mq-tag.c
>> @@ -23,9 +23,19 @@
>>    */
>>   bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
>>   {
>> -    if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
>> -        !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> -        atomic_inc(&hctx->tags->active_queues);
>> +    struct request_queue *q = hctx->queue;
>> +    struct blk_mq_tag_set *set = q->tag_set;
>> +
>> +    if (blk_mq_is_sbitmap_shared(set)){
>> +        if (!test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) &&
>> +            !test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
>> +            atomic_inc(&set->active_queues_shared_sbitmap);
>> +
>> +    } else {
>> +        if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
>> +            !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> +            atomic_inc(&hctx->tags->active_queues);
>> +    }
>>       return true;
>>   }
> At one point someone would need to educate me what this double
> 'test_bit' and 'test_and_set_bit' is supposed to achieve.
> Other than deliberately injecting a race condition ...

As I see, it's not a dangerous race, and meant as a performance
optimization.

test_bit() should be is quick compared to test_and_set_bit(), so nicer
to avoid always doing a test_and_set_bit().

So if we race with failed test_bit() calls and have multiple attempts
with the test_and_set_bit(), only 1 will succeed and do the increment.

>
>> @@ -47,11 +57,19 @@ void blk_mq_tag_wakeup_all(struct blk_mq_tags
>> *tags, bool include_reserve)
>>   void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
>>   {
>>       struct blk_mq_tags *tags = hctx->tags;
>> -
>> -    if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> -        return;
>> -
>> -    atomic_dec(&tags->active_queues);
>> +    struct request_queue *q = hctx->queue;
>> +    struct blk_mq_tag_set *set = q->tag_set;
>> +
>> +    if (blk_mq_is_sbitmap_shared(q->tag_set)){
>> +        if (!test_and_clear_bit(QUEUE_FLAG_HCTX_ACTIVE,
>> +                    &q->queue_flags))
>> +            return;
>> +        atomic_dec(&set->active_queues_shared_sbitmap);
>> +    } else {
>> +        if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> +            return;
>> +        atomic_dec(&tags->active_queues);
>> +    }
>>       blk_mq_tag_wakeup_all(tags, false);
>>   }
>> @@ -65,12 +83,11 @@ static inline bool hctx_may_queue(struct
>> blk_mq_alloc_data *data,
>>   {
>>       struct blk_mq_hw_ctx *hctx = data->hctx;
>>       struct request_queue *q = data->q;
>> +    struct blk_mq_tag_set *set = q->tag_set;
>>       unsigned int depth, users;
>>       if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
>>           return true;
>> -    if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> -        return true;
>>       /*
>>        * Don't try dividing an ant
>> @@ -78,7 +95,15 @@ static inline bool hctx_may_queue(struct
>> blk_mq_alloc_data *data,
>>       if (bt->sb.depth == 1)
>>           return true;
>> -    users = atomic_read(&hctx->tags->active_queues);
>> +    if (blk_mq_is_sbitmap_shared(q->tag_set)) {
>> +        if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &q->queue_flags))
>> +            return true;
>> +        users = atomic_read(&set->active_queues_shared_sbitmap);
>> +    } else {
>> +        if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>> +            return true;
>> +        users = atomic_read(&hctx->tags->active_queues);
>> +    }
>>       if (!users)
>>           return true;
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 0f7e062a1665..f73a2f9c58bd 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -3350,6 +3350,8 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set
>> *set)
>>           goto out_free_mq_map;
>>       if (blk_mq_is_sbitmap_shared(set)) {
>> +        atomic_set(&set->active_queues_shared_sbitmap, 0);
>> +
>>           if (!blk_mq_init_shared_sbitmap(set)) {
>>               ret = -ENOMEM;
>>               goto out_free_mq_rq_maps;
>> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
>> index 7b31cdb92a71..66711c7234db 100644
>> --- a/include/linux/blk-mq.h
>> +++ b/include/linux/blk-mq.h
>> @@ -252,6 +252,7 @@ struct blk_mq_tag_set {
>>       unsigned int        timeout;
>>       unsigned int        flags;
>>       void            *driver_data;
>> +    atomic_t        active_queues_shared_sbitmap;

I'm not sure if we should present this in debugfs. It is not according
to this series.

>>       struct sbitmap_queue    __bitmap_tags;
>>       struct sbitmap_queue    __breserved_tags;
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index c536278bec9e..1b0087e8d01a 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -619,6 +619,7 @@ struct request_queue {
>>   #define QUEUE_FLAG_PCI_P2PDMA    25    /* device supports PCI p2p
>> requests */
>>   #define QUEUE_FLAG_ZONE_RESETALL 26    /* supports Zone Reset All */
>>   #define QUEUE_FLAG_RQ_ALLOC_TIME 27    /* record rq->alloc_time_ns */
>> +#define QUEUE_FLAG_HCTX_ACTIVE 28    /* at least one blk-mq hctx is
>> active */
>>   #define QUEUE_FLAG_MQ_DEFAULT    ((1 << QUEUE_FLAG_IO_STAT) |        \
>>                    (1 << QUEUE_FLAG_SAME_COMP))
>>
> Other than that it looks fine.

Cheers