On Sat, Jun 23, 2018 at 6:19 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 6/22/18 4:12 PM, Ming Lei wrote:
>> SCSI probing may synchronously create and destroy a lot of request_queues
>> for non-existent devices. Any synchronize_rcu() in queue creation or
>> destroy path may introduce long latency during booting, see detailed
>> description in comment of blk_register_queue().
>>
>> This patch removes two synchronize_rcu() inside blk_cleanup_queue()
>> for this case:
>>
>> 1) commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue)
>> need synchronize_rcu() for implementing blk_mq_quiesce_queue(), but
>> when queue isn't initialized, it isn't necessary to do that since
>> only pass-through requests are involved, no original issue in
>> scsi_execute() at all.
>>
>> 2) when only one request queue is attached to tags, no necessary to
>> call synchronize_rcu() too.
>>
>> Without this patch, it may take more 20+ seconds for virtio-scsi to
>> complete disk probe. With this patch, the time becomes less than 100ms.
>>
>> Reported-by: Andrew Jones <drjones@xxxxxxxxxx>
>> Cc: Andrew Jones <drjones@xxxxxxxxxx>
>> Cc: linux-scsi@xxxxxxxxxxxxxxx
>> Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
>> ---
>> block/blk-core.c | 8 ++++++--
>> block/blk-mq.c | 5 ++++-
>> 2 files changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index cf0ee764b908..f0129e20b773 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -766,9 +766,13 @@ void blk_cleanup_queue(struct request_queue *q)
>> * make sure all in-progress dispatch are completed because
>> * blk_freeze_queue() can only complete all requests, and
>> * dispatch may still be in-progress since we dispatch requests
>> - * from more than one contexts
>> + * from more than one contexts.
>> + *
>> + * No need to quiesce queue if it isn't initialized yet since
>> + * blk_freeze_queue() should be enough for cases of passthrough
>> + * request.
>> */
>> - if (q->mq_ops)
>> + if (q->mq_ops && blk_queue_init_done(q))
>> blk_mq_quiesce_queue(q);
>>
>> /* for synchronous bio-based driver finish in-flight integrity i/o */
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 70c65bb6c013..8a6771ac0adb 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2351,6 +2351,7 @@ static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set,
>> static void blk_mq_del_queue_tag_set(struct request_queue *q)
>> {
>> struct blk_mq_tag_set *set = q->tag_set;
>> + bool shared;
>>
>> mutex_lock(&set->tag_list_lock);
>> list_del_rcu(&q->tag_set_list);
>> @@ -2360,8 +2361,10 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>> /* update existing queue */
>> blk_mq_update_tag_set_depth(set, false);
>> }
>> + shared = set->flags & BLK_MQ_F_TAG_SHARED;
>> mutex_unlock(&set->tag_list_lock);
>> - synchronize_rcu();
>> + if (shared)
>> + synchronize_rcu();
>
> Shouldn't this be set if it _was_ shared as well, not just if it's
> still shared?
Yes, it need to be done for _was_ shared. But for the usual single lun case,
the shared can be set for all the following probe suppose lun 0 is the real one,
then this simple trick can't work any more.
Looks blk_queue_init_done() still need to be check here.
thanks,
Ming Lei
