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Date: Thu, 29 Oct 2015 22:55:19 +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 10:20 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Hi Ming,
>
> Thanks for taking a look.
>
> Ming Lei <tom.leiming@...il.com> writes:
>
>> On Fri, Oct 23, 2015 at 10:57 PM, Jeff Moyer <jmoyer@...hat.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.
>
> Yes, it is possible. blk_mq_init_allocated_queue calls
> blk_mq_init_hw_queues without the set->tag_list_lock. Consider a single
> queue is present for the tag_set, and thus the BLK_MQ_F_TAG_SHARED flag
> is not set. Then, two more async scan events are running on separate
> CPUs. Each will go through this loop in blk_mq_init_hw_queues:
>
> queue_for_each_hw_ctx(q, hctx, i) {
> if (blk_mq_init_hctx(q, set, hctx, i))
> break;
> }
>
> and blk_mq_init_hctx copies the flags:
> hctx->flags = set->flags;
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.
>
> 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.
> hctx->flags were copied before the BLK_MQ_F_TAG_SHARED flag was set, it
> now will operate as if it's the only queue in the tag set.
The shared flag should be set here because the flag has been set in set->flags,
then no such issue, is there?
>
> So, yes, there is a race, and that extra check is necessary. Hopefully
> I've explained it well enough. If not, let me know and I'll try again.
>
> Cheers,
> Jeff
>
>>
>>> 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@...cle.com>
>>> Signed-off-by: Jeff Moyer <jmoyer@...hat.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
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