quoted

quoted

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