Re: [PATCH v4 01/14] svcrdma: Reduce the number of rdma_rw contexts per-QP

Jason Gunthorpe
2025-05-08T12:43:31Z

On Thu, May 08, 2025 at 11:41:18AM +0300, Edward Srouji wrote:
>
> On 5/6/2025 5:22 PM, Jason Gunthorpe wrote:
> > On Tue, May 06, 2025 at 10:19:06AM -0400, Chuck Lever wrote:
> > > > > In this patch I'm trying to include the reg/inv multiplier in the
> > > > > calculation, but that doesn't seem to be enough to make "accept"
> > > > > reliable, IMO due to this extra calculation in calc_sq_size().
> > > > Did ib_create_qp get called with more than max_qp_wr ?
> > > The request was for, like, 9300 SQEs. max_qp_wr is 32K on my systems.
> > Sounds like it is broken then..
> >
> > props->max_qp_wr = 1 << MLX5_CAP_GEN(mdev, log_max_qp_sz);
> >
> > So it is ignoring the wqe_size adustment.. It should adjust by the worst
> > case result of calc_send_wqe() for the device..
> How do you suggest adjusting to the worst case?
> How inline messages could be addressed and taken into account?

I think assume 0 size inline for computing max sizes

> Even if we ignore the inline size, worst case potentially could be less than
> 1/8th of the max HCA CAP, not sure we want to deliver this as a limitation
> to users.

The math is simply wrong - log_max_qp_sz is not the number of work
queue entries in the queue, it is the number of MLX5_SEND_WQE_BB's
units which is some internal value.

For a verbs API the result should be the max number of work queue
entries that can be requested for any of XRC/RC/UC/UD QP types using a
0 inline size, 1 SGL and no other special features.

Even for a simple RC QP sq_overhead() will return 132 which already
makes props->max_qp_wr uselessly wrong. 132 goes into here:

return ALIGN(max_t(int, inl_size, size), MLX5_SEND_WQE_BB);

Comes out as 192 - so props->max_qp_wr is off by 3x even for a simple
no-feature RC QP.

Chuck is getting:

calc_sq_size:618:(pid 1514): send queue size (9326 * 256 / 64 -> 65536) exceeds limits(32768)

So I suppose that extra 64 bytes is coming from cap.max_send_sge >= 3?

Without a new API we can't make it fully discoverable, but the way it
is now is clearly wrong.

Jason