sunrpc: skip svc_xprt_enqueue when no work is pending

svc_reserve() and svc_xprt_release_slot() call
svc_xprt_enqueue() after modifying xpt_reserved or
xpt_nr_rqsts. The purpose is to re-dispatch the
transport when write-space or a slot becomes available.
However, when neither XPT_DATA nor XPT_DEFERRED is
set, no thread can make progress on the transport and
the enqueue accomplishes nothing.

Trace data from a 256KB NFSv3 WRITE workload over RDMA
shows 11.2 svc_xprt_enqueue() calls per RPC. Of these,
6.9 per RPC lack XPT_DATA and exit svc_xprt_ready()
immediately after executing the smp_rmb(), READ_ONCE(),
and tracepoint. svc_reserve() and svc_xprt_release_slot()
account for roughly five of these per RPC.

A new helper, svc_xprt_resource_released(), checks
XPT_DATA | XPT_DEFERRED before calling
svc_xprt_enqueue(). The existing smp_wmb() barriers
are upgraded to smp_mb() to ensure the flags check
observes a concurrent producer's set_bit(XPT_DATA).
Each producer (svc_rdma_wc_receive, etc.) both sets
XPT_DATA and calls svc_xprt_enqueue(), so even if the
check reads a stale value, the producer's own enqueue
provides a fallback path.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

0 files changed, 0 insertions, 0 deletions