IB_POLL_DIRECT

[Bob Pearson <rpearsonhpe@gmail.com>]

Jason,

Both the rxe driver and the siw driver running the blktests srp test suite exhibit failures on my machine
running the for-next branch. This has been true for months so I decided to try again to track it down.
After a lot of tracing, it looks like the problem is that the built in cq handling in core/cq.c is failing to
continue to process some completion queues.

The traffic is between the srp driver and the srpt driver. The srpt driver uses

	cq = ib_cq_pool_get(..., IB_POLL_WORKQUEUE) and

the srp driver uses

	cq = ib_alloc_cq(..., IB_POLL_SOFTIRQ) for receive cqs and
	cq = ib_alloc_cq(..., IB_POLL_DIRECT) for send cqs.

AFAIK the poll workqueue and poll softirq cqs are working correctly but the poll direct cq sometimes
loses the thread and just stops processing those cqs. The test cases sometimes recover after about
a 2 second delay and start processing again and eventually fail after about a 10 second delay and
cleanup and go home.

The failures feel like a race or at least are timing sensitive. If you run the test suite several times
various test cases will sometimes succeed and sometimes fail. But they always fail in the same way.

Looking at the mlxn drivers for inspiration, I don't see anything specific about IB_POLL_DIRECT except
that they have a private version of send_queue_drain which also calls a cqe drain function which calls
ib_process_cq_direct() in a loop until the cq is drained. But this is only during qp tear down. (No other
verbs driver does this but as far as I know no other driver is passing blktests.) This is only done for
IB_POLL_DIRECT, so I wonder, is this required to use that correctly?

I am still figuring out how IB_POLL_DIRECT works. It doesn't allow the driver to call cq->comp_handler so
I don't know how it figures out when there are new wcs to process.

Any ideas would be really helpful.

Bob