Re: [patch] blk-mq: avoid excessive boot delays with large lun counts

[Ming Lei]

On Fri, Oct 23, 2015 at 10:57 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> Hi,
>
> Zhangqing Luo reported long boot times on a system with thousands of
> LUNs when scsi-mq was enabled.  He narrowed the problem down to
> blk_mq_add_queue_tag_set, where every queue is frozen in order to set
> the BLK_MQ_F_TAG_SHARED flag.  Each added device will freeze all queues
> added before it in sequence, which involves waiting for an RCU grace
> period for each one.  We don't need to do this.  After the second queue
> is added, only new queues need to be initialized with the shared tag.
> We can do that by percolating the flag up to the blk_mq_tag_set, since
> new hctxs already inherit the tag_set's flags.  That's what the below
> patch does.

Looks the idea is correct.

>
> The re-check in blk_mq_add_queue_tag_set is done because we could have
> two queues racing to be the second queue added for the tag set.  In such

That isn't possible because of set->tag_list_lock.

> a case, it's possible that not all of the hctxs within the loser will be
> initialized.  Because this initialization is done sequentially, we can
> simply check for the BLK_MQ_F_TAG_SHARED flag in the first hctx.
>
> This problem was introduced by commit 0d2602ca30e41 (blk-mq: improve
> support for shared tags maps).
>
> Reported-and-tested-by: Jason Luo <zhangqing.luo@xxxxxxxxxx>
> Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 7785ae9..8b4c484 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1860,27 +1860,26 @@ static void blk_mq_map_swqueue(struct request_queue *q,
>         }
>  }
>
> -static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set)
> +static void queue_set_hctx_shared(struct request_queue *q, bool shared)
>  {
>         struct blk_mq_hw_ctx *hctx;
> -       struct request_queue *q;
> -       bool shared;
>         int i;
>
> -       if (set->tag_list.next == set->tag_list.prev)
> -               shared = false;
> -       else
> -               shared = true;
> +       queue_for_each_hw_ctx(q, hctx, i) {
> +               if (shared)
> +                       hctx->flags |= BLK_MQ_F_TAG_SHARED;
> +               else
> +                       hctx->flags &= ~BLK_MQ_F_TAG_SHARED;
> +       }
> +}
> +
> +static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set, bool shared)
> +{
> +       struct request_queue *q;
>
>         list_for_each_entry(q, &set->tag_list, tag_set_list) {
>                 blk_mq_freeze_queue(q);
> -
> -               queue_for_each_hw_ctx(q, hctx, i) {
> -                       if (shared)
> -                               hctx->flags |= BLK_MQ_F_TAG_SHARED;
> -                       else
> -                               hctx->flags &= ~BLK_MQ_F_TAG_SHARED;
> -               }
> +               queue_set_hctx_shared(q, shared);
>                 blk_mq_unfreeze_queue(q);
>         }
>  }
> @@ -1891,7 +1890,13 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>
>         mutex_lock(&set->tag_list_lock);
>         list_del_init(&q->tag_set_list);
> -       blk_mq_update_tag_set_depth(set);
> +
> +       if (set->tag_list.next == set->tag_list.prev) {
> +               /* just transitioned to unshared */
> +               set->flags &= ~BLK_MQ_F_TAG_SHARED;
> +               /* update existing queue */
> +               blk_mq_update_tag_set_depth(set, false);
> +       }
>         mutex_unlock(&set->tag_list_lock);
>  }
>
> @@ -1902,7 +1907,24 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
>
>         mutex_lock(&set->tag_list_lock);
>         list_add_tail(&q->tag_set_list, &set->tag_list);
> -       blk_mq_update_tag_set_depth(set);
> +
> +       if (set->tag_list.next != set->tag_list.prev) {
> +               /*
> +                * Only update the tag set state if the state has
> +                * actually changed.
> +                */
> +               if (!(set->flags & BLK_MQ_F_TAG_SHARED)) {
> +                       /* just transitioned to shared tags */
> +                       set->flags |= BLK_MQ_F_TAG_SHARED;
> +                       blk_mq_update_tag_set_depth(set, true);
> +               } else {
> +                       /* ensure we didn't race with another addition */
> +                       struct blk_mq_hw_ctx *hctx = queue_first_hw_ctx(q);
> +                       if ((hctx->flags & BLK_MQ_F_TAG_SHARED) !=
> +                                                       BLK_MQ_F_TAG_SHARED)
> +                               queue_set_hctx_shared(q, true);

The above check isn't necessary because the queue can't be up now, so
it is OK to just call 'queue_set_hctx_shared(q, true)' directly.

> +               }
> +       }
>         mutex_unlock(&set->tag_list_lock);
>  }
>
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 5e7d43a..12ffc40 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -254,6 +254,9 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq)
>         for ((i) = 0; (i) < (hctx)->nr_ctx &&                           \
>              ({ ctx = (hctx)->ctxs[(i)]; 1; }); (i)++)
>
> +#define queue_first_hw_ctx(q)                                          \
> +       (q)->queue_hw_ctx[0]
> +
>  #define blk_ctx_sum(q, sum)                                            \
>  ({                                                                     \
>         struct blk_mq_ctx *__x;                                         \



-- 
Ming Lei
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