On 6/22/18 5:42 AM, Andrew Jones wrote:
> On Wed, Jun 20, 2018 at 10:55:22AM +0800, 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..63680b243466 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 = true;
>>
>> mutex_lock(&set->tag_list_lock);
>> list_del_rcu(&q->tag_set_list);
>> @@ -2359,9 +2360,11 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>> set->flags &= ~BLK_MQ_F_TAG_SHARED;
>> /* update existing queue */
>> blk_mq_update_tag_set_depth(set, false);
>> + shared = true;
>
> I guess this should be '= false'.
>
>> }
>> mutex_unlock(&set->tag_list_lock);
>> - synchronize_rcu();
>> + if (shared)
>> + synchronize_rcu();
>> INIT_LIST_HEAD(&q->tag_set_list);
>> }
>>
>
> With the '= false' change I tested this and it resolves the issue for me.
That logic still doesn't look correct to me. Does the below work?
diff --git a/block/blk-core.c b/block/blk-core.c
index afd2596ea3d3..222d4fc0e524 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -762,9 +762,13 @@ 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 8e57b84e50e9..18ad2b95ff63 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2351,8 +2351,12 @@ 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);
+
+ shared = !list_is_singular(&set->tag_list);
+
list_del_rcu(&q->tag_set_list);
if (list_is_singular(&set->tag_list)) {
/* just transitioned to unshared */
@@ -2361,7 +2365,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
blk_mq_update_tag_set_depth(set, false);
}
mutex_unlock(&set->tag_list_lock);
- synchronize_rcu();
+ if (shared)
+ synchronize_rcu();
INIT_LIST_HEAD(&q->tag_set_list);
}
--
Jens Axboe
