From: Chuck Lever Date: Tue, 24 Mar 2026 13:04:48 +0000 (-0400) Subject: sunrpc: skip svc_xprt_enqueue in svc_xprt_received when idle X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0a5d7be861782f760431363b172ef2575645c5e6;p=thirdparty%2Flinux.git 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 Signed-off-by: Chuck Lever --- diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c index b8f3ac29e7186..377f5d7490aa4 100644 --- a/net/sunrpc/svc_xprt.c +++ b/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);