[PATCH 0/7] Implied-ACK eviction for NFSD's duplicate reply cache

[Chuck Lever <cel@kernel.org>]

Connection-oriented transports (TCP, RDMA) guarantee in-order
delivery: arrival of a new request proves every prior reply on that
connection has been received. This series exploits that property to
evict completed DRC entries as soon as the next request arrives on
the same transport, rather than waiting out the 120-second
RC_EXPIRE timeout.

Each DRC entry records its originating transport. A new
xpt_last_recv timestamp on svc_xprt marks when the most recent
request arrived. During bucket pruning, nfsd_prune_bucket_locked()
compares the entry's timestamp against xpt_last_recv; a newer
value on the same transport means the reply was delivered, and the
entry is evicted immediately.

Profiled with "perf record -e cycles -e cpu-clock -e LLC-load-misses
-e branch-misses" during an NFSv3/RDMA 4KB random-write workload.
nfsd_cache_lookup overhead dropped from 1.53% to 0.76% of CPU
cycles. The rb-tree operations (rb_erase, rb_insert_color) that
dominated LLC cache misses fell from a combined 13.2% to 1.1% of all
LLC-load-misses, because shorter-lived entries keep the per-bucket
trees small.

---
Chuck Lever (7):
      SUNRPC: Assign a unique identifier to each svc_xprt
      NFSD: Track transport in DRC entries
      SUNRPC: Record last-request timestamp on svc_xprt
      SUNRPC: Mark connection-oriented transports on svc_xprt
      NFSD: Evict completed DRC entries via implied ACK
      NFSD: Add tracepoints for DRC entry eviction
      NFSD: Record DRC population in lookup tracepoints

 fs/nfsd/cache.h                          |  1 +
 fs/nfsd/nfscache.c                       | 88 +++++++++++++++++++++++++++-----
 fs/nfsd/trace.h                          | 49 ++++++++++++++++--
 include/linux/sunrpc/svc_xprt.h          |  5 +-
 include/trace/events/sunrpc.h            |  9 +++-
 net/sunrpc/netns.h                       |  4 ++
 net/sunrpc/sunrpc_syms.c                 |  3 ++
 net/sunrpc/svc_xprt.c                    | 54 ++++++++++++++++++--
 net/sunrpc/svcsock.c                     | 41 ++++++++++-----
 net/sunrpc/xprtrdma/svc_rdma_transport.c |  6 ++-
 10 files changed, 222 insertions(+), 38 deletions(-)
---
base-commit: 3a4d7c7c9466a07fa4176ce35180149709d92c22
change-id: 20260325-duplicate-reply-cache-f0fe7c7b740c

Best regards,
--  
Chuck Lever <cel@kernel.org>