From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E04B5241665 for ; Tue, 24 Mar 2026 13:04:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774357495; cv=none; b=JC6F5ywDXKCsqV32Zs5vF++WgVlwoWNngYo+nQbSKU6t3Wj0BujxEhVrEiWKRfJTZtJhpi2U946iwzhmPeGeF8wj8PcR0FA0pCC+i4UtE3dRwx351sGsTwBa1l4QsNJkPS/hklXGSh3qLgYyj9U4fKGzbyxip5Bb507vAu2kbeE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774357495; c=relaxed/simple; bh=ZQUCoTSOuetgU65z+aORZVWRuQ+TP43ReL9UCzmKKvI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KqghU/uLxmkE+G5jYoBHdkSr1eqCr03zfiGv5Jn49i7DAMntB92+C0DPY73WeyftUwaDm7V5yeISjcTzZztHHNBbyXgKjLc8J58AqWsfbWjpGGMbBLf17FgVUxjIEWPikKWB4oLS/sIhDOTT2VnwfgwnvVBPFHKfI0rskuQv2c4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TTxm1FQf; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TTxm1FQf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3073FC2BC87; Tue, 24 Mar 2026 13:04:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774357495; bh=ZQUCoTSOuetgU65z+aORZVWRuQ+TP43ReL9UCzmKKvI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=TTxm1FQfOFtNkBjQRTP2XkGwYNyC6vw3FevNua6TTvDshXqy4nO+KDlRNhxZGQbtu irpgg+ns3bukYoESPTZ/Jn5NCz+LdVm/hEbXbp1o0v3FvIIPRIA7PGDQGRj4s7pxNw PjOe+SY6QH33LLi8CMfM9vxbTFBHEg3outZlLSrcX8PfYSEuAVrZHf82vCemd6DMwY 0bvh2arADLOo+Hr2MI385xyzWqKAC2L3+7By9I04et3Yeugb1LSLa21/oBwFXDTi22 366QDFglZXNG6nTvNXIlUCzsU82CztPc7PM5s+jL5Qj0VbrisqevlDCfn7tvud2pmV CKJsnH+phzakg== From: Chuck Lever To: NeilBrown , Jeff Layton , Olga Kornievskaia , Dai Ngo , Tom Talpey Cc: , Chuck Lever Subject: [PATCH 2/3] sunrpc: skip svc_xprt_enqueue in svc_xprt_received when idle Date: Tue, 24 Mar 2026 09:04:48 -0400 Message-ID: <20260324130449.16437-3-cel@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260324130449.16437-1-cel@kernel.org> References: <20260324130449.16437-1-cel@kernel.org> Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Chuck Lever 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. Signed-off-by: Chuck Lever --- net/sunrpc/svc_xprt.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c index 73149280167c..36c8437cfd8d 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); -- 2.53.0