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 05925C433FF for ; Sun, 11 Aug 2019 23:30:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A30AC2084C for ; Sun, 11 Aug 2019 23:30:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725870AbfHKXaZ (ORCPT ); Sun, 11 Aug 2019 19:30:25 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:61702 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725855AbfHKXaZ (ORCPT ); Sun, 11 Aug 2019 19:30:25 -0400 Received: from pps.filterd (m0098413.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7BNR112055996; Sun, 11 Aug 2019 19:30:22 -0400 Received: from ppma05wdc.us.ibm.com (1b.90.2fa9.ip4.static.sl-reverse.com [169.47.144.27]) by mx0b-001b2d01.pphosted.com with ESMTP id 2uatsuask7-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 11 Aug 2019 19:30:22 -0400 Received: from pps.filterd (ppma05wdc.us.ibm.com [127.0.0.1]) by ppma05wdc.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7BNU4MQ012937; Sun, 11 Aug 2019 23:30:21 GMT Received: from b01cxnp22034.gho.pok.ibm.com (b01cxnp22034.gho.pok.ibm.com [9.57.198.24]) by ppma05wdc.us.ibm.com with ESMTP id 2u9nj617aa-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 11 Aug 2019 23:30:21 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7BNUL1C36766110 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sun, 11 Aug 2019 23:30:21 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7B57FB205F; Sun, 11 Aug 2019 23:30:21 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4790BB2065; Sun, 11 Aug 2019 23:30:21 +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 23:30:21 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 189D716C1B1A; Sun, 11 Aug 2019 16:30:24 -0700 (PDT) Date: Sun, 11 Aug 2019 16:30:24 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: rcu Subject: Re: need_heavy_qs flag for PREEMPT=y kernels Message-ID: <20190811233024.GZ28441@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190811180852.GA128944@google.com> <20190811211646.GY28441@linux.ibm.com> <20190811212505.GB128944@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190811212505.GB128944@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_12:,, 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-1908110262 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 11, 2019 at 05:25:05PM -0400, Joel Fernandes wrote: > On Sun, Aug 11, 2019 at 02:16:46PM -0700, Paul E. McKenney wrote: > > On Sun, Aug 11, 2019 at 02:34:08PM -0400, Joel Fernandes wrote: > > > On Sun, Aug 11, 2019 at 2:08 PM 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? > > > > > > Oh, so I take it this bit of code in rcu_implicit_dynticks_qs(), with > > > the accompanying comments, takes care of the scenario I describe? > > > Another way could be just call rcu_momentary_dyntick_idle() during > > > cond_resched() for nohz_full CPUs? Is that pricey? > > > /* > > > * NO_HZ_FULL CPUs can run in-kernel without rcu_sched_clock_irq! > > > * The above code handles this, but only for straight cond_resched(). > > > * And some in-kernel loops check need_resched() before calling > > > * cond_resched(), which defeats the above code for CPUs that are > > > * running in-kernel with scheduling-clock interrupts disabled. > > > * So hit them over the head with the resched_cpu() hammer! > > > */ > > > if (tick_nohz_full_cpu(rdp->cpu) && > > > time_after(jiffies, > > > READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) { > > > resched_cpu(rdp->cpu); > > > WRITE_ONCE(rdp->last_fqs_resched, jiffies); > > > } > > > > Yes, for NO_HZ_FULL=y&&PREEMPT=y kernels. > > Actually, I was only referring to the case of NO_HZ_FULL=y being the > troublesome one (i.e. rcu_need_heavy_qs flag would have no effect). > > For NO_HZ_FULL=n, I have full confidence the scheduler tick will notice > rcu_urgent_qs and do a reschedule. The ensuing softirq then does the needful > to help end the grace period. Whew! That confidence was not at all apparent in your initial email. > > Your thought of including rcu_momentary_dyntick_idle() would function > > correctly, but would cause performance issues. Even adding additional > > compares and branches in that hot codepath is visible to 0day test robot! > > So adding a read-modify-write atomic operation to that code path would > > get attention of the wrong kind. ;-) > > But wouldn't these performance issues also be visible with > NO_HZ_FULL=y && PREEMPT=n? In PREEMPT=n, cond_resched() already has a check, and with quite a bit of care it is possible to introduce another. > Why is PREEMPT=n made an exception? The exception is actually CONFIG_NO_HZ_FULL=y && CONFIG_PREEMPT=y. In that case, we can rely on neither the scheduling-clock interrupt nor on cond_resched(). In the other three cases, we have one or both. > Is it that > 0day doesn't test this combination much? :-D Might be, but it sure tests the other combinations! Next question: Why does rcu_implicit_dynticks_qs() check only for tick_nohz_full_cpu() and not also IS_ENABLED(CONFIG_PREEMPT)? After all, a nohz_full CPU in a !CONFIG_PREEMPT kernel should be able to rely on cond_resched(), right? Should this change? Why or why not? Thanx, Paul