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

[Hannes Reinecke <hare@suse.de>]

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 <wenxiong@us.ibm.com>
> Cc: John Garry <john.garry@huawei.com>
> 
> 
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
hare@suse.de                              +49 911 74053 688
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