[PATCH v2 0/2] improve nvme quiesce time for large amount of namespaces

[Chao Leng]

Now nvme_stop_queues quiesce all queues one by one. Every queue must
wait a grace period(rcu or srcu). If the controller has a large amount
of namespaces, the total waiting time is very long.
Test result: the total waiting time is more than 20 seconds when the
controller has 256 namespace.

This set improves the quiesce time when using a large set of namespaces,
which also improves I/O failover time in a multipath environment.

We improve for both non-blocking tagset and blocking tagset introducing
blk_mq_[un]quiesce_tagset which works on all request queues over a given
tagset in parallel (which is the case in nvme).

Changes from v2:
- replace call_srcu to start_poll_synchronize_srcu
- rename the flag name to make it accurate.
- move unquiescing queue outside from nvme_set_queue_dying
- add mutex to ensure that all queues are quiesced after set the NVME_CTRL_STOPPED.

Changes from v1:
- improvement is based on tagset rather than namesapces

Chao Leng (2):
  blk-mq: add tagset quiesce interface
  nvme: use blk_mq_[un]quiesce_tagset

 block/blk-mq.c           | 76 ++++++++++++++++++++++++++++++++++++++++++++++++
 drivers/nvme/host/core.c | 57 +++++++++++++++---------------------
 drivers/nvme/host/nvme.h |  3 +-
 include/linux/blk-mq.h   |  2 ++
 include/linux/blkdev.h   |  3 ++
 5 files changed, 106 insertions(+), 35 deletions(-)

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2.16.4