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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,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 0A8B7C46470 for ; Wed, 8 Aug 2018 14:42:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BA38521737 for ; Wed, 8 Aug 2018 14:42:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BA38521737 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 S1727361AbeHHRCH (ORCPT ); Wed, 8 Aug 2018 13:02:07 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:42304 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726954AbeHHRCG (ORCPT ); Wed, 8 Aug 2018 13:02:06 -0400 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w78EdoSl062723 for ; Wed, 8 Aug 2018 10:42:08 -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 2kr08c6kq5-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 08 Aug 2018 10:42:08 -0400 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 8 Aug 2018 10:42:05 -0400 Received: from b01cxnp23032.gho.pok.ibm.com (9.57.198.27) by e16.ny.us.ibm.com (146.89.104.203) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 8 Aug 2018 10:42:00 -0400 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 w78Efx6216843144 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 8 Aug 2018 14:41:59 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B3899B2065; Wed, 8 Aug 2018 10:41:23 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8240DB205F; Wed, 8 Aug 2018 10:41:23 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 8 Aug 2018 10:41:23 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8096516C0727; Wed, 8 Aug 2018 07:42:00 -0700 (PDT) Date: Wed, 8 Aug 2018 07:42:00 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Joel Fernandes , Joel Fernandes , LKML , "Cc: Android Kernel" , Boqun Feng , Byungchul Park , Ingo Molnar , Masami Hiramatsu , Mathieu Desnoyers , Namhyung Kim , Peter Zijlstra , Thomas Glexiner , Tom Zanussi Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage Reply-To: paulmck@linux.vnet.ibm.com References: <6D0A3FD6-2190-4CC0-A3C0-7B3759E73243@google.com> <20180807204820.50b83c6d@vmware.local.home> <20180807215522.04114097@vmware.local.home> <20180807222856.3ede96e7@vmware.local.home> <20180808130041.GI24813@linux.vnet.ibm.com> <20180808102700.38c4169d@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180808102700.38c4169d@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18080814-0072-0000-0000-0000038D74EE X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009507; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01070983; UDB=6.00551365; IPR=6.00850513; MB=3.00022589; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-08 14:42:03 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18080814-0073-0000-0000-000049026778 Message-Id: <20180808144200.GM24813@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-08_05:,, 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=8.0.1-1807170000 definitions=main-1808080152 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 08, 2018 at 10:27:00AM -0400, Steven Rostedt wrote: > On Wed, 8 Aug 2018 06:00:41 -0700 > "Paul E. McKenney" wrote: > > > > I suppose that an srcu_read_lock_nmi() and srcu_read_unlock_nmi() could > > be added, which would do atomic ops on sp->sda->srcu_lock_count. Not sure > > whether this would be fast enough to be useful, but easy to provide: > > > > int __srcu_read_lock_nmi(struct srcu_struct *sp) /* UNTESTED. */ > > { > > int idx; > > > > idx = READ_ONCE(sp->srcu_idx) & 0x1; > > atomic_inc(&sp->sda->srcu_lock_count[idx]); > > smp_mb__after_atomic(); /* B */ /* Avoid leaking critical section. */ > > return idx; > > } > > > > void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx) > > { > > smp_mb__before_atomic(); /* C */ /* Avoid leaking critical section. */ > > atomic_inc(&sp->sda->srcu_unlock_count[idx]); > > } > > > > With appropriate adjustments to also allow Tiny RCU to also work. > > > > Note that you have to use _nmi() everywhere, not just in NMI handlers. > > In fact, the NMI handlers are the one place you -don't- need to use > > _nmi(), strangely enough. > > > > Might be worth a try -- smp_mb__{before,after}_atomic() is a no-op on > > some architectures, for example. > > Note this would kill the performance that srcu gives that Joel wants. > Switching from a this_cpu_inc() to a atomic_inc() would be a huge > impact. I don't know how huge it would be, but that concern is exactly why I am proposing adding _nmi() interfaces rather than just directly changing the stock __srcu_read_lock() and __srcu_read_unlock() functions. > There's also a local_inc() if you are using per cpu pointers, that is > suppose to guarantee atomicity for single cpu operations. That's what > the ftrace ring buffer uses. Good point, that becomes atomic_long_inc() or equivalent on most architectures, but an incl instruction (not locked) on x86. So updating my earlier still-untested thought: int __srcu_read_lock_nmi(struct srcu_struct *sp) /* UNTESTED. */ { int idx; idx = READ_ONCE(sp->srcu_idx) & 0x1; local_inc(&sp->sda->srcu_lock_count[idx]); smp_mb__after_atomic(); /* B */ /* Avoid leaking critical section. */ return idx; } void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx) { smp_mb__before_atomic(); /* C */ /* Avoid leaking critical section. */ local_inc(&sp->sda->srcu_unlock_count[idx]); } Would that work, or is there a better way to handle this? Thanx, Paul