From patchwork Wed Jul 29 08:42:28 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Schoenebeck X-Patchwork-Id: 11710715 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 165E2138C for ; Wed, 12 Aug 2020 12:46:43 +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 E0D522065C for ; Wed, 12 Aug 2020 12:46:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=crudebyte.com header.i=@crudebyte.com header.b="bUaJfQqL" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E0D522065C 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]:59988 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1k5q9R-0000wk-SE for patchwork-qemu-devel@patchwork.kernel.org; Wed, 12 Aug 2020 08:46:41 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35384) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k5q8n-000881-77 for qemu-devel@nongnu.org; Wed, 12 Aug 2020 08:46:01 -0400 Received: from lizzy.crudebyte.com ([91.194.90.13]:56265) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k5q8l-0005fu-Mx for qemu-devel@nongnu.org; Wed, 12 Aug 2020 08:46:00 -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=iIOY7lnyCREAN0i5cfmgv+fsotclpHEmOG5eXJ6R0b4=; b=bUaJf QqLeGuM9Nl6kcIvVz0EEH5Oa0xRx+FlHzuB9DFykCI4ANl5Uvsa73OSIjgwfzvT3ZjwH37R0XjZHJ Lnv/kP2hHWh6fn2Cm8J2owNIRQZNMJl800RtiTUSP2DKq1w5O7m3eXCohEiIVI0i0QYlHDFXaTazo /kRqFBTh+MAHE81gSNgFW87uyIm5ybsd/ZwwYrbOYR9Gw+jeeO8egdhFhXSyuTi3P1VzXPocia8xU t+kjAGG2Hq6x18AWCrSLVslOAIXzfvj8ihdleTcuKipmE1iX4iU5W5Tq+QYBuTIvB9WuyXOz/7MHb l1wuu9xx1N+u+eXSdpN8O63AThyxQ==; Message-Id: In-Reply-To: References: From: Christian Schoenebeck Date: Wed, 29 Jul 2020 10:42:28 +0200 Subject: [PULL 7/7] 9pfs: clarify latency of v9fs_co_run_in_worker() To: qemu-devel@nongnu.org, Peter Maydell Cc: Greg Kurz Received-SPF: none client-ip=91.194.90.13; envelope-from=da9f2eda2551c1cbd98f72730e5b754f2149a85c@lizzy.crudebyte.com; helo=lizzy.crudebyte.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/12 08:16:56 X-ACL-Warn: Detected OS = Linux 3.11 and newer X-Spam_score_int: 13 X-Spam_score: 1.3 X-Spam_bar: + X-Spam_report: (1.3 / 5.0 requ) BAYES_00=-1.9, DATE_IN_PAST_96_XX=3.405, 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=no 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 Message-Id: <4d34d332e1aaa8a2cf8dc0b5da4fd7727f2a86e8.1596012787.git.qemu_oss@crudebyte.com> 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 { \