From patchwork Sun Jul 19 13:39:59 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Schoenebeck X-Patchwork-Id: 11672475 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 5F8E414E3 for ; Sun, 19 Jul 2020 14:40:56 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 350F72080D for ; Sun, 19 Jul 2020 14:40:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=crudebyte.com header.i=@crudebyte.com header.b="eVweJzS+" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 350F72080D Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=crudebyte.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Received: from localhost ([::1]:60098 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jxAUp-0004Yh-HL for patchwork-qemu-devel@patchwork.kernel.org; Sun, 19 Jul 2020 10:40:55 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40004) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jxAUJ-00042n-Dr for qemu-devel@nongnu.org; Sun, 19 Jul 2020 10:40:23 -0400 Received: from lizzy.crudebyte.com ([91.194.90.13]:45411) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jxAUH-0005Cv-Jf for qemu-devel@nongnu.org; Sun, 19 Jul 2020 10:40:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=crudebyte.com; s=lizzy; h=Cc:To:Subject:Date:From:References:In-Reply-To: Message-Id:Content-Type:Content-Transfer-Encoding:MIME-Version:Content-ID: Content-Description; bh=dniHciHWD1s2sUmY62gnL597ZGnuF2fTUNk4k5tqqLE=; b=eVweJ zS+kwjAOSABAGZ/OnBEdXfzmO7BfwNaxrKRKbK5BUgXgL1NzbhbmncITp9StAj9qBulqV0BpBWzLv 5HAnIib+Dq7tjnXlQS418i5pjYly96pPyGySSN7JOGDXnjZUkHSlEJWbAZkvfYU+kfXp2sE3RQAKq tsdDh4DmxK1Hap2XvDyDNWhfR9J7SFEbTLcUdo6tna80k3i5WKiL1TbUL4h5fRhaz8kImqlbn5WsX GPCyrL0kGeLYrqaD90Om0yjPYSKtJ4GOvjHH6NgV4xwt7Sw2/OJ7CPLEfTcVpXQSAuw0hK2Fzz0gw 4SRkF0Ak8qPXBzKx80FjXGCyyk3Pw==; Message-Id: In-Reply-To: References: From: Christian Schoenebeck Date: Sun, 19 Jul 2020 15:39:59 +0200 Subject: [PATCH v7 6/6] 9pfs: clarify latency of v9fs_co_run_in_worker() To: qemu-devel@nongnu.org Cc: Greg Kurz Received-SPF: none client-ip=91.194.90.13; envelope-from=cb67ffe2c8e5bddc31511d62a859cebfda0b7feb@lizzy.crudebyte.com; helo=lizzy.crudebyte.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/19 10:12:15 X-ACL-Warn: Detected OS = Linux 3.11 and newer X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_NONE=0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" As we just fixed a severe performance issue with Treaddir request handling, clarify this overall issue as a comment on v9fs_co_run_in_worker() with the intention to hopefully prevent such performance mistakes in future (and fixing other yet outstanding ones). Signed-off-by: Christian Schoenebeck --- hw/9pfs/coth.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h index fd4a45bc7c..c51289903d 100644 --- a/hw/9pfs/coth.h +++ b/hw/9pfs/coth.h @@ -19,7 +19,7 @@ #include "qemu/coroutine.h" #include "9p.h" -/* +/** * we want to use bottom half because we want to make sure the below * sequence of events. * @@ -28,6 +28,16 @@ * 3. Enter the coroutine in the worker thread. * we cannot swap step 1 and 2, because that would imply worker thread * can enter coroutine while step1 is still running + * + * @b PERFORMANCE @b CONSIDERATIONS: As a rule of thumb, keep in mind + * that hopping between threads adds @b latency! So when handling a + * 9pfs request, avoid calling v9fs_co_run_in_worker() too often, because + * this might otherwise sum up to a significant, huge overall latency for + * providing the response for just a single request. For that reason it + * is highly recommended to fetch all data from fs driver with a single + * fs driver request on a background I/O thread (bottom half) in one rush + * first and then eventually assembling the final response from that data + * on main I/O thread (top half). */ #define v9fs_co_run_in_worker(code_block) \ do { \