[ATTEND] scsi-mq prototype discussion

[Nicholas A. Bellinger]

Hello,

I would like to attend the 2013 Kernel Summit.

At the summit, I would like to discuss scsi-mq, a high performance SCSI
initiator prototype that utilizes the next-generation blk-mq effort by
Jens Axboe.  The long-term goal is a path to move beyond the
long-standing small block random I/O limitations vs. raw make_request
based drivers of the existing Linux/SCSI client stack.

Along with using blk-mq's excellent native per-cpu primitive + NUMA
local friendly queuing of pre-allocated struct request descriptor
memory, the scsi-mq prototype currently avoids all I/O fast-path access
of legacy scsi_host->host_lock, and bypasses existing scsi_request_fn()
dispatch into scsi-mq enabled LLD code.

It also allows scsi-core to eliminate all fast-path memory allocations
using struct scsi_cmnd + $LLD_CMD pre-allocations based on a per struct
blk_mq_hw_ctx -> scsi_device->sdev_mq_req context, along with per
scsi_cmnd descriptor pre-allocation of SGL and sense buffer memory.

So far the initial conversion of virtio-scsi + scsi-debug LLDs has been
completed.  Also, the intention is to keep the conversion requirements
for existing LLDs to scsi-mq as simple as possible.

There are still many areas that have been conveniently left out of the
initial prototype, including proper fast-path get_device() +
put_device() reference counting, a functioning scsi-generic IOCTL,
anything close to per struct scsi_device error handling, amongst other
things..

Drilling down the work items ahead of a real mainline push is high on
priority list for discussion.

The parties to be included in such a discussion are:

  - Jens Axboe (blk-mq author)
  - James Bottomley (scsi maintainer)
  - Christoph Hellwig (scsi)
  - Martin Petersen (scsi)
  - Tejun Heo (block + libata)
  - Hannes Reinecke (scsi error recovery)
  - Kent Overstreet (block, per-cpu ida)
  - Stephen Cameron (scsi-over-pcie driver)
  - Andrew Vasquez (qla2xxx LLD)
  - James Smart (lpfc LLD)

Thank you,

--nab

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