Re: blk-mq: takes hours for scsi scanning finish when thousands of LUNs

[Tejun Heo <tj@kernel.org>]

Hello,

On Mon, Oct 19, 2015 at 07:40:13AM -0700, Zhangqing Luo wrote:
....
> So every time blk_mq_freeze_queue_start, it runs in this way
> 
> blk_mq_freeze_queue_start
> ->percpu_ref_kill->percpu_ref_kill_and_confirm
> ->__percpu_ref_switch_to_atomic
> ->call_rcu_sched(&ref->rcu,percpu_ref_switch_to_atomic_rcu)
> 
> and blk_mq_freeze_queue_wait blocks on queue->mq_usage_counter
> as it is not zero, and wake up by percpu_ref_switch_to_atomic_rcu
> after a grace period
> 
> 
> My question here is why should we change ref to PERCPU at blk_mq_finish_init?
> because of this changing, delay appears.

Because percpu operation is way cheaper than atomic ones and we want
to optimize hot paths (request issue and completion) over cold paths
(init and config changes).  That's the whole point of percpu
refcnting.

The reason why percpu ref starts in atomic mode is to avoid expensive
percpu freezing if the queue is created and abandoned in quick
succession as SCSI does during LUN scanning.  If percpu freezing is
happening during that, the right solution is moving finish_init to
late enough point so that percpu switching happens only after it's
known that the queue won't be abandoned.

Thanks.

-- 
tejun