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Chuck Lever 7051a80916 svcrdma: Reduce the number of rdma_rw contexts per-QP 
[ Upstream commit 5924331589 ]

There is an upper bound on the number of rdma_rw contexts that can
be created per QP.

This invisible upper bound is because rdma_create_qp() adds one or
more additional SQEs for each ctxt that the ULP requests via
qp_attr.cap.max_rdma_ctxs. The QP's actual Send Queue length is on
the order of the sum of qp_attr.cap.max_send_wr and a factor times
qp_attr.cap.max_rdma_ctxs. The factor can be up to three, depending
on whether MR operations are required before RDMA Reads.

This limit is not visible to RDMA consumers via dev->attrs. When the
limit is surpassed, QP creation fails with -ENOMEM. For example:

svcrdma's estimate of the number of rdma_rw contexts it needs is
three times the number of pages in RPCSVC_MAXPAGES. When MAXPAGES
is about 260, the internally-computed SQ length should be:

64 credits + 10 backlog + 3 * (3 * 260) = 2414

Which is well below the advertised qp_max_wr of 32768.

If RPCSVC_MAXPAGES is increased to 4MB, that's 1040 pages:

64 credits + 10 backlog + 3 * (3 * 1040) = 9434

However, QP creation fails. Dynamic printk for mlx5 shows:

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

Although 9326 is still far below qp_max_wr, QP creation still
fails.

Because the total SQ length calculation is opaque to RDMA consumers,
there doesn't seem to be much that can be done about this except for
consumers to try to keep the requested rdma_rw ctxt count low.

Fixes: 2da0f610e7 ("svcrdma: Increase the per-transport rw_ctx count")
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-06-19 15:31:39 +02:00
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README

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.