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=-1.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URG_BIZ,USER_AGENT_MUTT 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 83E2BC6778A for ; Mon, 9 Jul 2018 14:27:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 47D532087F for ; Mon, 9 Jul 2018 14:27:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 47D532087F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.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 S932967AbeGIO1w (ORCPT ); Mon, 9 Jul 2018 10:27:52 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:51258 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750996AbeGIO1v (ORCPT ); Mon, 9 Jul 2018 10:27:51 -0400 Received: from pps.filterd (m0098417.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w69ENkOc117398 for ; Mon, 9 Jul 2018 10:27:50 -0400 Received: from e13.ny.us.ibm.com (e13.ny.us.ibm.com [129.33.205.203]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k48jg3x54-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 09 Jul 2018 10:27:50 -0400 Received: from localhost by e13.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 9 Jul 2018 10:27:49 -0400 Received: from b01cxnp22035.gho.pok.ibm.com (9.57.198.25) by e13.ny.us.ibm.com (146.89.104.200) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 9 Jul 2018 10:27:46 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w69ERknX5833286 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 9 Jul 2018 14:27:46 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3A9C7B2076; Mon, 9 Jul 2018 10:27:21 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 26ACEB2072; Mon, 9 Jul 2018 10:27:21 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 9 Jul 2018 10:27:21 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 448DB16C0824; Mon, 9 Jul 2018 07:30:04 -0700 (PDT) Date: Mon, 9 Jul 2018 07:30:04 -0700 From: "Paul E. McKenney" To: David Woodhouse Cc: Peter Zijlstra , mhillenb@amazon.de, linux-kernel Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <20180706162905.GZ2476@hirez.programming.kicks-ass.net> <20180706171150.GI3593@linux.vnet.ibm.com> <20180709085351.GC2476@hirez.programming.kicks-ass.net> <1531127935.18697.57.camel@infradead.org> <20180709104429.GI2476@hirez.programming.kicks-ass.net> <1531133801.18697.73.camel@infradead.org> <20180709110657.GL2476@hirez.programming.kicks-ass.net> <20180709123457.GM3593@linux.vnet.ibm.com> <1531140434.18697.82.camel@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1531140434.18697.82.camel@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070914-0064-0000-0000-000003274FCC X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009338; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01058854; UDB=6.00543380; IPR=6.00836775; MB=3.00022073; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-09 14:27:48 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070914-0065-0000-0000-000039E00CBF Message-Id: <20180709143004.GP3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-09_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=613 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807090165 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 09, 2018 at 01:47:14PM +0100, David Woodhouse wrote: > On Mon, 2018-07-09 at 05:34 -0700, Paul E. McKenney wrote: > > The reason that David's latencies went from 100ms to one second is > > because I made this code less aggressive about invoking resched_cpu(). > > Ten seconds. We saw synchronize_sched() take ten seconds in 4.15. We > wouldn't have been happy with one second, but ten seconds was > considered particularly suboptimal. Yes, ten seconds. Please accept my apologies for my early morning confusion. Thanx, Paul > > The reason I did that was to allow cond_resched_rcu_qs() to be used less > > without performance regressions.  And just plain cond_resched() on > > !PREEMPT is intended to handle the faster checks.  But KVM defeats > > this by checking need_resched() before invoking cond_resched(). > > It isn't just KVM. It's a relatively common construct to use > need_resched(), then drop any local locks around cond_resched(). > > A bare cond_resched() will call rcu_all_qs() unconditionally, and it is > kind of inconsistent that need_resched() doesn't include the > corresponding condition.