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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 22129C0650F for ; Sun, 11 Aug 2019 21:13:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EBAC8208C2 for ; Sun, 11 Aug 2019 21:13:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726053AbfHKVNU (ORCPT ); Sun, 11 Aug 2019 17:13:20 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:29284 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726014AbfHKVNU (ORCPT ); Sun, 11 Aug 2019 17:13:20 -0400 Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7BL6bvT018018; Sun, 11 Aug 2019 17:13:17 -0400 Received: from ppma01wdc.us.ibm.com (fd.55.37a9.ip4.static.sl-reverse.com [169.55.85.253]) by mx0a-001b2d01.pphosted.com with ESMTP id 2uaqynkwn4-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 11 Aug 2019 17:13:16 -0400 Received: from pps.filterd (ppma01wdc.us.ibm.com [127.0.0.1]) by ppma01wdc.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7BL4d34003417; Sun, 11 Aug 2019 21:13:16 GMT Received: from b01cxnp23032.gho.pok.ibm.com (b01cxnp23032.gho.pok.ibm.com [9.57.198.27]) by ppma01wdc.us.ibm.com with ESMTP id 2u9nj6abm9-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 11 Aug 2019 21:13:16 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7BLDGOX40894858 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sun, 11 Aug 2019 21:13:16 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id ED263B2068; Sun, 11 Aug 2019 21:13:15 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id CD306B2064; Sun, 11 Aug 2019 21:13:15 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.138.198]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 11 Aug 2019 21:13:15 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 7BB2516C170F; Sun, 11 Aug 2019 14:13:18 -0700 (PDT) Date: Sun, 11 Aug 2019 14:13:18 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: rcu@vger.kernel.org Subject: Re: need_heavy_qs flag for PREEMPT=y kernels Message-ID: <20190811211318.GX28441@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190811180852.GA128944@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190811180852.GA128944@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-11_10:,, 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=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908110236 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 11, 2019 at 02:08:52PM -0400, Joel Fernandes wrote: > Hi Paul, everyone, > > I noticed on reading code that the need_heavy_qs check and > rcu_momentary_dyntick_idle() is only called for !PREEMPT kernels. Don't we > need to call this for PREEMPT kernels for the benefit of nohz_full CPUs? > > Consider the following events: > 1. Kernel is PREEMPT=y configuration. > 2. CPU 2 is a nohz_full CPU running only a single task and the tick is off. > 3. CPU 2 is running only in kernel mode and does not enter user mode or idle. > 4. Grace period thread running on CPU 3 enter the fqs loop. > 5. Enough time passes and it sets the need_heavy_qs for CPU2. > 6. CPU 2 is still in kernel mode but does cond_resched(). > 7. cond_resched() does not call rcu_momentary_dyntick_idle() because PREEMPT=y. > > Is 7. not calling rcu_momentary_dyntick_idle() a lost opportunity for the FQS > loop to detect that the CPU has crossed a quiescent point? > > Is this done so that cond_resched() is fast for PREEMPT=y kernels? The problem is that the definiton of cond_resched() in PREEMPT=y kernels is as follows: static inline int _cond_resched(void) { return 0; } If (but only if!) someone shows a problem in a PREEMPT=y kernel, the code could be updated to do something like a resched_cpu() earlier rather than later. The reason that I do not expect a problem in NO_HZ_FULL=n&&PREEMPT=y kernels is that the scheduling-clock tick will with high probability happen when the CPU is not in an RCU read-side critical section, and this quiescent state will be reported reasonably quickly. This leaves NO_HZ_FULL=y&&PREEMPT=y kernels. In that case, RCU is more aggressive about using resched_cpu() on CPUs that have not yet reported a quiescent state for the current grace period. So we should be good as is. Or am I missing a key corner case here? Thanx, Paul