Re: [PATCH net-next v3] net/smc: transition to RDMA core CQ pooling

On Thu, Mar 05, 2026 at 10:23:23AM +0800, D. Wythe wrote:

The current SMC-R implementation relies on global per-device CQs
and manual polling within tasklets, which introduces severe
scalability bottlenecks due to global lock contention and tasklet
scheduling overhead, resulting in poor performance as concurrency
increases.

Refactor the completion handling to utilize the ib_cqe API and
standard RDMA core CQ pooling. This transition provides several key
advantages:

1. Multi-CQ: Shift from a single shared per-device CQ to multiple
link-specific CQs via the CQ pool. This allows completion processing
to be parallelized across multiple CPU cores, effectively eliminating
the global CQ bottleneck.

2. Leverage DIM: Utilizing the standard CQ pool with IB_POLL_SOFTIRQ
enables Dynamic Interrupt Moderation from the RDMA core, optimizing
interrupt frequency and reducing CPU load under high pressure.

3. O(1) Context Retrieval: Replaces the expensive wr_id based lookup
logic (e.g., smc_wr_tx_find_pending_index) with direct context retrieval
using container_of() on the embedded ib_cqe.

4. Code Simplification: This refactoring results in a reduction of
~150 lines of code. It removes redundant sequence tracking, complex lookup
helpers, and manual CQ management, significantly improving maintainability.

Performance Test: redis-benchmark with max 32 connections per QP
Data format: Requests Per Second (RPS), Percentage in brackets
represents the gain/loss compared to TCP.

| Clients | TCP      | SMC (original)      | SMC (cq_pool)       |
|---------|----------|---------------------|---------------------|
| c = 1   | 24449    | 31172  (+27%)       | 34039  (+39%)       |
| c = 2   | 46420    | 53216  (+14%)       | 64391  (+38%)       |
| c = 16  | 159673   | 83668  (-48%)  <--  | 216947 (+36%)       |
| c = 32  | 164956   | 97631  (-41%)  <--  | 249376 (+51%)       |
| c = 64  | 166322   | 118192 (-29%)  <--  | 249488 (+50%)       |
| c = 128 | 167700   | 121497 (-27%)  <--  | 249480 (+48%)       |
| c = 256 | 175021   | 146109 (-16%)  <--  | 240384 (+37%)       |
| c = 512 | 168987   | 101479 (-40%)  <--  | 226634 (+34%)       |

The results demonstrate that this optimization effectively resolves the
scalability bottleneck, with RPS increasing by over 110% at c=64
compared to the original implementation.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
---
v3:
- Rebase to latest net-next tree.
- Remove a redundant blank line in smc_wr_alloc_link_mem().

v2:
- Fix a logic bug in smc_wr_tx_process_cqe() where a zeroed field
  was checked instead of the saved pnd_snd copy. (Jakub)
- Fix typo in comment: s/ib_draib_rq/ib_drain_rq/.
- Minor comment alignment fix in struct smc_link.
---
 net/smc/smc_core.c |   9 +-
 net/smc/smc_core.h |  28 ++--
 net/smc/smc_ib.c   | 113 +++++-----------
 net/smc/smc_ib.h   |   7 -
 net/smc/smc_tx.c   |   1 -
 net/smc/smc_wr.c   | 312 +++++++++++++++++++--------------------------
 net/smc/smc_wr.h   |  40 ++----
 7 files changed, 193 insertions(+), 317 deletions(-)

Thanks a lot for this important conversion, for RDMA API usage:

Reviewed-by: Leon Romanovsky <leonro@nvidia.com>