>>> It might make sense to lock it separately, but I would not worry
>>> about it unless it shows up as hot in your testing.
>> Actually, we've met a triggering of nmi_watchdog, blocked at the queue lock
>> in blkcg_print_blkgs(), caused by the slow serial console and too many printks.
>> Related discussion is here,
>> https://bugzilla.kernel.org/show_bug.cgi?id=199003
>> Even though it's not caused by the queue lock directly, it would not happen
>> without using queue lock. The queue lock is big and used too widely, using it
>> would intensify the race, so we're trying to understand the locks using in blkg,
>> and maybe could improve the situation.
> 
> The queue lock is only used widely on non blk-mq, where it is the only
> lock really. Doing serial IO under a spinlock is always going to suck,
> regardless of how contended it is.

It's not a problem of queue lock indeed, just want to dig deeper. :) 
Thanks for the reply.

Jiang,
Regards