[PATCH V3 0/5] blk-mq: improvement on handling IO during CPU hotplug

[Ming Lei <ming.lei@redhat.com>]

Hi,

Thomas mentioned:
    "
     That was the constraint of managed interrupts from the very beginning:
    
      The driver/subsystem has to quiesce the interrupt line and the associated
      queue _before_ it gets shutdown in CPU unplug and not fiddle with it
      until it's restarted by the core when the CPU is plugged in again.
    "

But no drivers or blk-mq do that before one hctx becomes dead(all
CPUs for one hctx are offline), and even it is worse, blk-mq stills tries
to run hw queue after hctx is dead, see blk_mq_hctx_notify_dead().

This patchset tries to address the issue by two stages:

1) add one new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE

- mark the hctx as internal stopped, and drain all in-flight requests
if the hctx is going to be dead.

2) re-submit IO in the state of CPUHP_BLK_MQ_DEAD after the hctx becomes dead

- steal bios from the request, and resubmit them via generic_make_request(),
then these IO will be mapped to other live hctx for dispatch

Please comment & review, thanks!

John, I don't add your tested-by tag since V3 have some changes,
and I appreciate if you may run your test on V3.

V3:
	- re-organize patch 2 & 3 a bit for addressing Hannes's comment
	- fix patch 4 for avoiding potential deadlock, as found by Hannes

V2:
	- patch4 & patch 5 in V1 have been merged to block tree, so remove
	  them
	- address comments from John Garry and Minwoo


Ming Lei (5):
  blk-mq: add new state of BLK_MQ_S_INTERNAL_STOPPED
  blk-mq: prepare for draining IO when hctx's all CPUs are offline
  blk-mq: stop to handle IO and drain IO before hctx becomes dead
  blk-mq: re-submit IO in case that hctx is dead
  blk-mq: handle requests dispatched from IO scheduler in case that hctx
    is dead

 block/blk-mq-debugfs.c     |   2 +
 block/blk-mq-tag.c         |   2 +-
 block/blk-mq-tag.h         |   2 +
 block/blk-mq.c             | 135 +++++++++++++++++++++++++++++++++----
 block/blk-mq.h             |   3 +-
 drivers/block/loop.c       |   2 +-
 drivers/md/dm-rq.c         |   2 +-
 include/linux/blk-mq.h     |   5 ++
 include/linux/cpuhotplug.h |   1 +
 9 files changed, 138 insertions(+), 16 deletions(-)

Cc: John Garry <john.garry@huawei.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
-- 
2.20.1