Received: by 10.213.65.68 with SMTP id h4csp1336869imn; Mon, 26 Mar 2018 05:46:43 -0700 (PDT) X-Google-Smtp-Source: AIpwx48L7iEZ2rACdwI3NxEZ5H91nc6wQuKmFpdIuNO+p1zwR0ly9H735xrfFLiaEX2aPlkHwsUl X-Received: by 10.98.238.3 with SMTP id e3mr2446353pfi.88.1522068403468; Mon, 26 Mar 2018 05:46:43 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1522068403; cv=none; d=google.com; s=arc-20160816; b=Mi2lXJv9v1Kp0e4KFF01IxWOJBPEWk0WJHmfFGCSLbBTUxvIjDFmooHKi4lCFShzmh d0zB8E6rs1wFqFfVCc/Uijsv3SfdbZ5XVSn2LNuDwlLkwkT1sN8HOtCBojPWMYazA/6P GV7fuknwp8Sw2TJTOLbH4witH6aQ+WfXkWMKHqzOrYFTbQzsJ9Qy8drE3qU/1KylDXIB 0mN8kINaiHGzrOI2hNMjsoY9bDU+x5ih3wY7TqAoc1pnoskkFea96uhUL5KMpZCuQG9Q +wqFIf22rOSVdJ/ouqojorMUepwEC5dIl4KXuDOzkBeFkUlEOkoqJmAZ52F5cRcnk4zL FOHw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:message-id:user-agent:in-reply-to :content-disposition:mime-version:references:reply-to:subject:cc:to :from:date:arc-authentication-results; bh=mPG8KfhKN7ml2sPFB05NGjs1gUoMQUDpCRX88w+hMRk=; b=pz7wKoak1Pp0YojM11ATK94vQZ81M92IqhAC3kYAhk4X2A2QcYUdD3IheBsDZ4YBP+ lmTMK4mOBoy4uSWyzDeNF/SbcdR7AWifRzN/BuvzhAJpocQdDccZV9MFJoHABncB3o5W E4h6umrRyaoZFoSoKcg4FHApcB9tjrekYPIvTQl5RC4SoxzcUryhW/mH2icOwGJLIT9u T+rQJfvSlGyVA+aNh5DHKvB8DXUgkhEUwJ2ZV1a+h/OFJAtnyGCFiAC7W50rcwgT+EiT jf5ERJsJXYSc/rU9HFDEK7UbKXGMhr85d3qeI6mUrromWZvRnwSPFpLlctKWngNGaSCQ S9sw== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=ibm.com Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id p29si10143537pgn.123.2018.03.26.05.46.28; Mon, 26 Mar 2018 05:46:43 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=ibm.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752008AbeCZMpM (ORCPT + 99 others); Mon, 26 Mar 2018 08:45:12 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51016 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751355AbeCZMpL (ORCPT ); Mon, 26 Mar 2018 08:45:11 -0400 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w2QCgtjp038250 for ; Mon, 26 Mar 2018 08:45:11 -0400 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0a-001b2d01.pphosted.com with ESMTP id 2gxwwmrx08-1 (version=TLSv1.2 cipher=AES256-SHA256 bits=256 verify=NOT) for ; Mon, 26 Mar 2018 08:45:10 -0400 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 26 Mar 2018 08:45:09 -0400 Received: from b01cxnp23033.gho.pok.ibm.com (9.57.198.28) by e16.ny.us.ibm.com (146.89.104.203) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Mon, 26 Mar 2018 08:45:03 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w2QCj3HT59506760; Mon, 26 Mar 2018 12:45:03 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7F09EB2050; Mon, 26 Mar 2018 09:47:13 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.183.129]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP id 4C8A2B204E; Mon, 26 Mar 2018 09:47:13 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 63B2416C213C; Mon, 26 Mar 2018 05:45:55 -0700 (PDT) Date: Mon, 26 Mar 2018 05:45:55 -0700 From: "Paul E. McKenney" To: Yury Norov Cc: Chris Metcalf , Christopher Lameter , Russell King - ARM Linux , Mark Rutland , Steven Rostedt , Mathieu Desnoyers , Catalin Marinas , Will Deacon , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, luto@kernel.org Subject: Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync() Reply-To: paulmck@linux.vnet.ibm.com References: <20180325175004.28162-1-ynorov@caviumnetworks.com> <20180325175004.28162-3-ynorov@caviumnetworks.com> <20180325192328.GI3675@linux.vnet.ibm.com> <20180325201154.icdcyl4nw2jootqq@yury-thinkpad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180325201154.icdcyl4nw2jootqq@yury-thinkpad> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18032612-0024-0000-0000-0000033BE95E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008747; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000255; SDB=6.01008663; UDB=6.00513761; IPR=6.00787945; MB=3.00020248; MTD=3.00000008; XFM=3.00000015; UTC=2018-03-26 12:45:09 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18032612-0025-0000-0000-0000477313AA Message-Id: <20180326124555.GJ3675@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-03-26_06:,, 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 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1803260137 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 25, 2018 at 11:11:54PM +0300, Yury Norov wrote: > On Sun, Mar 25, 2018 at 12:23:28PM -0700, Paul E. McKenney wrote: > > On Sun, Mar 25, 2018 at 08:50:04PM +0300, Yury Norov wrote: > > > kick_all_cpus_sync() forces all CPUs to sync caches by sending broadcast IPI. > > > If CPU is in extended quiescent state (idle task or nohz_full userspace), this > > > work may be done at the exit of this state. Delaying synchronization helps to > > > save power if CPU is in idle state and decrease latency for real-time tasks. > > > > > > This patch introduces kick_active_cpus_sync() and uses it in mm/slab and arm64 > > > code to delay syncronization. > > > > > > For task isolation (https://lkml.org/lkml/2017/11/3/589), IPI to the CPU running > > > isolated task would be fatal, as it breaks isolation. The approach with delaying > > > of synchronization work helps to maintain isolated state. > > > > > > I've tested it with test from task isolation series on ThunderX2 for more than > > > 10 hours (10k giga-ticks) without breaking isolation. > > > > > > Signed-off-by: Yury Norov > > > --- > > > arch/arm64/kernel/insn.c | 2 +- > > > include/linux/smp.h | 2 ++ > > > kernel/smp.c | 24 ++++++++++++++++++++++++ > > > mm/slab.c | 2 +- > > > 4 files changed, 28 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c > > > index 2718a77da165..9d7c492e920e 100644 > > > --- a/arch/arm64/kernel/insn.c > > > +++ b/arch/arm64/kernel/insn.c > > > @@ -291,7 +291,7 @@ int __kprobes aarch64_insn_patch_text(void *addrs[], u32 insns[], int cnt) > > > * synchronization. > > > */ > > > ret = aarch64_insn_patch_text_nosync(addrs[0], insns[0]); > > > - kick_all_cpus_sync(); > > > + kick_active_cpus_sync(); > > > return ret; > > > } > > > } > > > diff --git a/include/linux/smp.h b/include/linux/smp.h > > > index 9fb239e12b82..27215e22240d 100644 > > > --- a/include/linux/smp.h > > > +++ b/include/linux/smp.h > > > @@ -105,6 +105,7 @@ int smp_call_function_any(const struct cpumask *mask, > > > smp_call_func_t func, void *info, int wait); > > > > > > void kick_all_cpus_sync(void); > > > +void kick_active_cpus_sync(void); > > > void wake_up_all_idle_cpus(void); > > > > > > /* > > > @@ -161,6 +162,7 @@ smp_call_function_any(const struct cpumask *mask, smp_call_func_t func, > > > } > > > > > > static inline void kick_all_cpus_sync(void) { } > > > +static inline void kick_active_cpus_sync(void) { } > > > static inline void wake_up_all_idle_cpus(void) { } > > > > > > #ifdef CONFIG_UP_LATE_INIT > > > diff --git a/kernel/smp.c b/kernel/smp.c > > > index 084c8b3a2681..0358d6673850 100644 > > > --- a/kernel/smp.c > > > +++ b/kernel/smp.c > > > @@ -724,6 +724,30 @@ void kick_all_cpus_sync(void) > > > } > > > EXPORT_SYMBOL_GPL(kick_all_cpus_sync); > > > > > > +/** > > > + * kick_active_cpus_sync - Force CPUs that are not in extended > > > + * quiescent state (idle or nohz_full userspace) sync by sending > > > + * IPI. Extended quiescent state CPUs will sync at the exit of > > > + * that state. > > > + */ > > > +void kick_active_cpus_sync(void) > > > +{ > > > + int cpu; > > > + struct cpumask kernel_cpus; > > > + > > > + smp_mb(); > > > + > > > + cpumask_clear(&kernel_cpus); > > > + preempt_disable(); > > > + for_each_online_cpu(cpu) { > > > + if (!rcu_eqs_special_set(cpu)) > > > > If we get here, the CPU is not in a quiescent state, so we therefore > > must IPI it, correct? > > > > But don't you also need to define rcu_eqs_special_exit() so that RCU > > can invoke it when it next leaves its quiescent state? Or are you able > > to ignore the CPU in that case? (If you are able to ignore the CPU in > > that case, I could give you a lower-cost function to get your job done.) > > > > Thanx, Paul > > What's actually needed for synchronization is issuing memory barrier on target > CPUs before we start executing kernel code. > > smp_mb() is implicitly called in smp_call_function*() path for it. In > rcu_eqs_special_set() -> rcu_dynticks_eqs_exit() path, smp_mb__after_atomic() > is called just before rcu_eqs_special_exit(). > > So I think, rcu_eqs_special_exit() may be left untouched. Empty > rcu_eqs_special_exit() in new RCU path corresponds empty do_nothing() in old > IPI path. > > Or my understanding of smp_mb__after_atomic() is wrong? By default, > smp_mb__after_atomic() is just alias to smp_mb(). But some > architectures define it differently. x86, for example, aliases it to > just barrier() with a comment: "Atomic operations are already > serializing on x86". > > I was initially thinking that it's also fine to leave > rcu_eqs_special_exit() empty in this case, but now I'm not sure... > > Anyway, answering to your question, we shouldn't ignore quiescent > CPUs, and rcu_eqs_special_set() path is really needed as it issues > memory barrier on them. An alternative approach would be for me to make something like this and export it: bool rcu_cpu_in_eqs(int cpu) { struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); int snap; smp_mb(); /* Obtain consistent snapshot, pairs with update. */ snap = READ_ONCE(&rdtp->dynticks); smp_mb(); /* See above. */ return !(snap & RCU_DYNTICK_CTRL_CTR); } Then you could replace your use of rcu_cpu_in_eqs() above with the new rcu_cpu_in_eqs(). This would avoid the RMW atomic, and, more important, the unnecessary write to ->dynticks. Or am I missing something? Thanx, Paul > Yury > > > > + cpumask_set_cpu(cpu, &kernel_cpus); > > > + } > > > + smp_call_function_many(&kernel_cpus, do_nothing, NULL, 1); > > > + preempt_enable(); > > > +} > > > +EXPORT_SYMBOL_GPL(kick_active_cpus_sync); > > > + > > > /** > > > * wake_up_all_idle_cpus - break all cpus out of idle > > > * wake_up_all_idle_cpus try to break all cpus which is in idle state even > > > diff --git a/mm/slab.c b/mm/slab.c > > > index 324446621b3e..678d5dbd6f46 100644 > > > --- a/mm/slab.c > > > +++ b/mm/slab.c > > > @@ -3856,7 +3856,7 @@ static int __do_tune_cpucache(struct kmem_cache *cachep, int limit, > > > * cpus, so skip the IPIs. > > > */ > > > if (prev) > > > - kick_all_cpus_sync(); > > > + kick_active_cpus_sync(); > > > > > > check_irq_on(); > > > cachep->batchcount = batchcount; > > > -- > > > 2.14.1 > > > >