Re: [PATCH for-rc v1] RDMA/rxe: Avoid CQ polling hang triggered by CQ resize

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Aug 27, 2025 at 08:14:53PM +0900, Daisuke Matsuda wrote:
> On 2025/08/26 3:10, Jason Gunthorpe wrote:
> > On Sun, Aug 17, 2025 at 12:37:52PM +0000, Daisuke Matsuda wrote:
> > > When running the test_resize_cq testcase from rdma-core, polling a
> > > completion queue from userspace may occasionally hang and eventually fail
> > > with a timeout:
> > > =====
> > > ERROR: test_resize_cq (tests.test_cq.CQTest.test_resize_cq)
> > > Test resize CQ, start with specific value and then increase and decrease
> > > ----------------------------------------------------------------------
> > > Traceback (most recent call last):
> > >      File "/root/deb/rdma-core/tests/test_cq.py", line 135, in test_resize_cq
> > >        u.poll_cq(self.client.cq)
> > >      File "/root/deb/rdma-core/tests/utils.py", line 687, in poll_cq
> > >        wcs = _poll_cq(cq, count, data)
> > >              ^^^^^^^^^^^^^^^^^^^^^^^^^
> > >      File "/root/deb/rdma-core/tests/utils.py", line 669, in _poll_cq
> > >        raise PyverbsError(f'Got timeout on polling ({count} CQEs remaining)')
> > > pyverbs.pyverbs_error.PyverbsError: Got timeout on polling (1 CQEs
> > > remaining)
> > > =====
> > > 
> > > The issue is caused when rxe_cq_post() fails to post a CQE due to the queue
> > > being temporarily full, and the CQE is effectively lost. To mitigate this,
> > > add a bounded busy-wait with fallback rescheduling so that CQE does not get
> > > lost.
> > 
> > Nothing should spin like this, that is not right.
> > 
> > The CQE queue is intended to be properly sized for the workload and I
> > seem to remember overflowing a CQE can just ERR the QP.
> > 
> > Alternatively you can drop the packet and do nothing.
> > 
> > But not spin hoping something emptys it.
> > 
> > Jason
> 
> Okay, please drop this patch.
> In a sense, the failure indicates that RXE is behaving as intended.
> 
> This issue seems to have always existed, though its frequency appears to vary over time.
> Perhaps the switch from tasklet to workqueue introduced additional latency that influences this behavior.

I'm not sure the test suite should be hitting cq overflows though, it
should have properly sized cqs for what it is doing..

Something sounds wrong.

Jason