Re: [PATCH 0/2] blk-mq: fix blk_mq_alloc_request_hctx

On 6/29/21 9:49 AM, Ming Lei wrote:

Hi,

blk_mq_alloc_request_hctx() is used by NVMe fc/rdma/tcp/loop to connect
io queue. Also the sw ctx is chosen as the 1st online cpu in hctx->cpumask.
However, all cpus in hctx->cpumask may be offline.

This usage model isn't well supported by blk-mq which supposes allocator is
always done on one online CPU in hctx->cpumask. This assumption is
related with managed irq, which also requires blk-mq to drain inflight
request in this hctx when the last cpu in hctx->cpumask is going to
offline.

However, NVMe fc/rdma/tcp/loop don't use managed irq, so we should allow
them to ask for request allocation when the specified hctx is inactive
(all cpus in hctx->cpumask are offline).

Fix blk_mq_alloc_request_hctx() by adding/passing flag of
BLK_MQ_F_NOT_USE_MANAGED_IRQ.


Ming Lei (2):
   blk-mq: not deactivate hctx if the device doesn't use managed irq
   nvme: pass BLK_MQ_F_NOT_USE_MANAGED_IRQ for fc/rdma/tcp/loop

  block/blk-mq.c             | 6 +++++-
  drivers/nvme/host/fc.c     | 3 ++-
  drivers/nvme/host/rdma.c   | 3 ++-
  drivers/nvme/host/tcp.c    | 3 ++-
  drivers/nvme/target/loop.c | 3 ++-
  include/linux/blk-mq.h     | 1 +
  6 files changed, 14 insertions(+), 5 deletions(-)

Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Daniel Wagner <dwagner@suse. thede>
Cc: Wen Xiong <redacted>
Cc: John Garry <redacted>

I have my misgivings about this patchset.
To my understanding, only CPUs present in the hctx cpumask are eligible to submit I/O to that hctx.
Consequently if all cpus in that mask are offline, where is the point of even transmitting a 'connect' request?
Shouldn't we rather modify the tagset to only refer to the current online CPUs _only_, thereby never submit a connect request for hctx with only offline CPUs?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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