On Fri, Oct 23, 2015 at 10:57 PM, Jeff Moyer <jmo...@redhat.com> 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....@oracle.com> > Signed-off-by: Jeff Moyer <jmo...@redhat.com> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/