From patchwork Tue May 24 20:06:14 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Emilio Cota X-Patchwork-Id: 9134239 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 3461D6075E for ; Tue, 24 May 2016 20:11:12 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 2A581281F9 for ; Tue, 24 May 2016 20:11:12 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 1BB87282A0; Tue, 24 May 2016 20:11:12 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=2.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI,T_DKIM_INVALID autolearn=ham version=3.3.1 Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 91BC728220 for ; Tue, 24 May 2016 20:11:11 +0000 (UTC) Received: from localhost ([::1]:55193 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5IfV-0008VV-3P for patchwork-qemu-devel@patchwork.kernel.org; Tue, 24 May 2016 16:11:09 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37783) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5Ib4-0004ZL-AI for qemu-devel@nongnu.org; Tue, 24 May 2016 16:06:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b5Iax-0004f0-NG for qemu-devel@nongnu.org; Tue, 24 May 2016 16:06:33 -0400 Received: from out2-smtp.messagingengine.com ([66.111.4.26]:49230) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b5Iav-0004d1-EP for qemu-devel@nongnu.org; Tue, 24 May 2016 16:06:27 -0400 Received: from compute6.internal (compute6.nyi.internal [10.202.2.46]) by mailout.nyi.internal (Postfix) with ESMTP id 9F548208CE; Tue, 24 May 2016 16:06:16 -0400 (EDT) Received: from frontend2 ([10.202.2.161]) by compute6.internal (MEProxy); Tue, 24 May 2016 16:06:16 -0400 DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; d=braap.org; h=cc :date:from:in-reply-to:message-id:references:subject:to :x-sasl-enc:x-sasl-enc; s=mesmtp; bh=gIs7Vurd5Ek3H/TnkjvMzrNpBrU =; b=SIM83KtNyB4EMaoU+L4LGU823zrT8cPb1g6872tP0yKhSNx7SCU2/3QdDTU /iZH/cB/ghy5lt78Z+KwJx1Ec7VL3+YaHYsfABs0dfGV1BGPv1hMA7gQkbETxZUH iZd2jI5fFyd9XWIr6WxY+kOQoLzAoZMHNEIzhu6WRc5LomLQ= DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; d= messagingengine.com; h=cc:date:from:in-reply-to:message-id :references:subject:to:x-sasl-enc:x-sasl-enc; s=smtpout; bh=gIs7 Vurd5Ek3H/TnkjvMzrNpBrU=; b=jqG1BYfRqArMUowQWenAJEp+VOXIhW86WQ7j 5AUF1nJUcFQJ/TbhCYgJRxYmdCKt3j5wsFRFZYnbAepS/3TIBjCtZv8cufXSQNf+ EAIWLtBkMg+0Yk7yfMxfvFGVxqRNXzDqEoDbb/l9wANBjzajRnxNPQgDwqCaMzGd 5qTI0Hc= X-Sasl-enc: RSeVH3JHtUsmWD/wgmpQGLEOHb2NPd6KhE2yJi0zo26l 1464120376 Received: from localhost (flamenco.cs.columbia.edu [128.59.20.216]) by mail.messagingengine.com (Postfix) with ESMTPA id 4D644CCD2E; Tue, 24 May 2016 16:06:16 -0400 (EDT) From: "Emilio G. Cota" To: QEMU Developers , MTTCG Devel Date: Tue, 24 May 2016 16:06:14 -0400 Message-Id: <1464120374-8950-4-git-send-email-cota@braap.org> X-Mailer: git-send-email 2.5.0 In-Reply-To: <1464120374-8950-1-git-send-email-cota@braap.org> References: <1464120374-8950-1-git-send-email-cota@braap.org> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 66.111.4.26 Subject: [Qemu-devel] [PATCH v2 3/3] atomics: do not emit consume barrier for atomic_rcu_read X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Sergey Fedorov , =?UTF-8?q?Alex=20Benn=C3=A9e?= , Richard Henderson Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP Currently we emit a consume-load in atomic_rcu_read. This is overkill for non-Sparc hosts, and is only useful to make things easier for Thread Sanitizer, which as far as I understand works best without explicit fences. The appended leaves the consume-load in atomic_rcu_read when compiling with Thread Sanitizer enabled, and resorts to a relaxed load + smp_read_barrier_depends otherwise. On an RMO host architecture, such as aarch64, the performance improvement of this change is easily measurable. For instance, qht-bench performs an atomic_rcu_read on every lookup. Performance before and after applying this patch: $ tests/qht-bench -d 5 -n 1 Before: 9.78 MT/s After: 10.96 MT/s Signed-off-by: Emilio G. Cota --- include/qemu/atomic.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index 4a4f2fb..c5b6c8d 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -63,13 +63,20 @@ __atomic_store(ptr, &_val, __ATOMIC_RELAXED); \ } while(0) -/* Atomic RCU operations imply weak memory barriers */ +#ifdef __SANITIZE_THREAD__ +#define atomic_rcu_read__nocheck(ptr, valptr) \ + __atomic_load(ptr, valptr, __ATOMIC_CONSUME); +#else +#define atomic_rcu_read__nocheck(ptr, valptr) \ + __atomic_load(ptr, valptr, __ATOMIC_RELAXED); \ + smp_read_barrier_depends(); +#endif #define atomic_rcu_read(ptr) \ ({ \ QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \ typeof(*ptr) _val; \ - __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \ + atomic_rcu_read__nocheck(ptr, &_val); \ _val; \ })