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Message-ID: <CACVXFVMADNe1qwMK5Wg7T8_FNWm7U_u5WAtcEyM6CTA3H_1dMg@mail.gmail.com> Date: Fri, 30 Oct 2015 08:37:05 +0800 From: Ming Lei <tom.leiming@...il.com> To: Jeff Moyer <jmoyer@...hat.com> Cc: Jens Axboe <axboe@...nel.dk>, Jason Luo <zhangqing.luo@...cle.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Guru Anbalagane <guru.anbalagane@...cle.com>, Feng Jin <joe.jin@...cle.com>, Tejun Heo <tj@...nel.org> Subject: Re: [patch] blk-mq: avoid excessive boot delays with large lun counts On Thu, Oct 29, 2015 at 11:18 PM, Jeff Moyer <jmoyer@...hat.com> wrote: > Ming Lei <tom.leiming@...il.com> writes: > >> Looks we should have cleared the TAG_SHARED flag during >> blk_mq_init_hctx() and just let blk_mq_update_tag_set_depth() >> deal with that, then the race can be avoided. > > The whole point of the patch set is to propagate the flag up to the tag > set so that we can avoid iterating all hctxs in all queues. Yes, the idea is correct, and the difference is that I don't see there isn't any race, for which your original patch tryed to address. > >>> At this point, neither queue's hctxs have the shared flag set. Next, >>> both will race to get the tag_list_lock for the tag_set inside of >>> blk_mq_add_queue_tag_set. Only one will win and mark the initial >>> queue's hctx's as shared (as well as its own). Then, when the second >>> queue gets the lock, it will find that the shared tag is already set, >>> and assume that it doesn't have to do anything. But, because its >> >> As I suggested, we can set it always in case that TAG_SHARED >> is set in set->flags because we know the queue isn't ready yet at that >> time. > > I see. You are suggesting that I just get rid of the conditional. We > could do that, but you will get the exact same result as what I posted. > I'm not sure why you would prefer that over the explicit check. With > the patch I posted, we can avoid walking the list of hctxs a second > time. > > Anyway, here's a patch that I think implements your suggestion. I > prefer the original, but this should achieve the same exact result. > Let me know if I've misunderstood. Yes, this one is better, and the big difference is that there isn't so called race, and the new one is more clean. > > Cheers, > Jeff > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 85f0143..7bf717a 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, > } > } The following change should be put into blk_mq_init_hctx() too. hctx->flags = set->flags & ~BLK_MQ_F_TAG_SHARED; and the idea is that we always figure out the shared flag in following blk_mq_update_tag_set_depth(). > > -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 +1907,21 @@ 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 */ The above comment should be removed, and I don't see there is any race now. > + queue_set_hctx_shared(q, true); > + } > + } > mutex_unlock(&set->tag_list_lock); > } > This one is a bit more clean than original one, and the trick of queue_first_hw_ctx() can be avoided. -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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