sunrpc: skip svc_xprt_enqueue in svc_xprt_received when idle

svc_xprt_received() unconditionally calls
svc_xprt_enqueue() after clearing XPT_BUSY. When no
work flags are pending, the enqueue traverses
svc_xprt_ready() -- executing an smp_rmb(), READ_ONCE(),
and tracepoint -- before returning false.

Trace data from a 256KB NFSv3 workload over RDMA shows
85% of svc_xprt_received() invocations reach
svc_xprt_enqueue() with no pending work flags. In the
WRITE phase, 167,335 of 196,420 calls find no work; in
the READ phase, 97,165 of 98,276. Each unnecessary call
executes a memory barrier, a flags read, and (when
tracing is active) fires the svc_xprt_enqueue
tracepoint.

Add a flags pre-check between clear_bit(XPT_BUSY) and
svc_xprt_enqueue(). Both the clear and the subsequent
READ_ONCE operate on the same xpt_flags word, so
cache-line serialization of the atomic bitops ensures
the read observes any flag set by a concurrent producer
before the line was acquired for the clear. If a
producer's set_bit occurs after the clear_bit, that
producer's own svc_xprt_enqueue() call observes
!XPT_BUSY and dispatches the transport.

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

Author: chucklever

net/sunrpc/svc_xprt.c

@@ -234,7 +234,19 @@ void svc_xprt_received(struct svc_xprt *xprt)
 	svc_xprt_get(xprt);
 	smp_mb__before_atomic();
 	clear_bit(XPT_BUSY, &xprt->xpt_flags);
-	svc_xprt_enqueue(xprt);
+
+	/*
+	 * Skip the enqueue when no actionable flags are set.
+	 * Each producer both sets its flag (XPT_DATA, XPT_CLOSE,
+	 * etc.) and calls svc_xprt_enqueue(); if a set_bit races
+	 * with this check, the producer's own enqueue observes
+	 * !XPT_BUSY and dispatches the transport.
+	 */
+	if (READ_ONCE(xprt->xpt_flags) &
+	    (BIT(XPT_CONN) | BIT(XPT_CLOSE) | BIT(XPT_HANDSHAKE) |
+	     BIT(XPT_DATA) | BIT(XPT_DEFERRED)))
+		svc_xprt_enqueue(xprt);
+
 	svc_xprt_put(xprt);
 }
 EXPORT_SYMBOL_GPL(svc_xprt_received);