From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8434DC43441 for ; Sun, 11 Nov 2018 19:57:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 523FE20871 for ; Sun, 11 Nov 2018 19:57:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 523FE20871 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729676AbeKLFrM (ORCPT ); Mon, 12 Nov 2018 00:47:12 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:54374 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727220AbeKLFqW (ORCPT ); Mon, 12 Nov 2018 00:46:22 -0500 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wABJs70l055519 for ; Sun, 11 Nov 2018 14:56:53 -0500 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0b-001b2d01.pphosted.com with ESMTP id 2npdbnt2jj-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Sun, 11 Nov 2018 14:56:53 -0500 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, kernel-team@android.com, "Paul E . McKenney" Subject: [PATCH tip/core/rcu 02/20] doc: Clarify RCU data-structure comment about rcu_tree fanout Date: Sun, 11 Nov 2018 11:56:27 -0800 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20181111195619.GA6958@linux.ibm.com> References: <20181111195619.GA6958@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18111119-0072-0000-0000-000003C6F417 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010028; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116023; UDB=6.00577118; IPR=6.00896124; MB=3.00024114; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-11 19:56:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111119-0073-0000-0000-00004A134C06 Message-Id: <20181111195645.9205-2-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811110190 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Joel Fernandes (Google)" RCU Data-Structures document describes a trick to test RCU with small number of CPUs but with a taller tree. It wasn't immediately clear how the document arrived at 16 CPUs which also requires setting the FANOUT_LEAF to 2 instead of the default of 16. This commit therefore provides the needed clarification. Signed-off-by: Joel Fernandes (Google) Cc: Signed-off-by: Paul E. McKenney --- .../RCU/Design/Data-Structures/Data-Structures.html | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html index 1d2051c0c3fc..476b1ac38e4c 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -127,9 +127,11 @@ CPUs, RCU would configure the rcu_node tree as follows:

RCU currently permits up to a four-level tree, which on a 64-bit system accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for 32-bit systems. -On the other hand, you can set CONFIG_RCU_FANOUT to be -as small as 2 if you wish, which would permit only 16 CPUs, which -is useful for testing. +On the other hand, you can set both CONFIG_RCU_FANOUT and +CONFIG_RCU_FANOUT_LEAF to be as small as 2, which would result +in a 16-CPU test using a 4-level tree. +This can be useful for testing large-system capabilities on small test +machines.

This multi-level combining tree allows us to get most of the performance and scalability -- 2.17.1