From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934406AbdDGPMS (ORCPT ); Fri, 7 Apr 2017 11:12:18 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:45926 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934046AbdDGPL4 (ORCPT ); Fri, 7 Apr 2017 11:11:56 -0400 Date: Fri, 7 Apr 2017 08:11:46 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton Subject: Re: [PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks() and stack_tracer_disable() Reply-To: paulmck@linux.vnet.ibm.com References: <20170407140106.051135969@goodmis.org> <20170407144334.GT1600@linux.vnet.ibm.com> <20170407105826.562b2e24@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170407105826.562b2e24@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17040715-0008-0000-0000-000001EA7F6D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006893; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00844316; UDB=6.00416175; IPR=6.00622613; BA=6.00005275; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00014953; XFM=3.00000013; UTC=2017-04-07 15:11:47 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17040715-0009-0000-0000-00003475F967 Message-Id: <20170407151146.GW1600@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-07_12:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1702020001 definitions=main-1704070126 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 07, 2017 at 10:58:26AM -0400, Steven Rostedt wrote: > On Fri, 7 Apr 2017 07:43:35 -0700 > "Paul E. McKenney" wrote: > > > On Fri, Apr 07, 2017 at 10:01:06AM -0400, Steven Rostedt wrote: > > > Paul, > > > > > > Here's my latest. You OK with it? > > > > Given your update to 3/5, I suspect that we could live with it. I am > > expecting some complaints about increases in idle-entry latency, but might > > be best to wait for complaints rather than complexifying too proactively. > > We only added a this_cpu_inc() and this_cpu_dec() which are very fast > operations. I highly doubt it will be measurable. Although, I'm talking > about x86, IIRC, the this_cpu_inc/dec were be poorly written for other > archs in the past. I'm not sure if that was fixed though. That is an issue for CPUs that don't have a to-memory increment instruction. How about __this_cpu_inc() and __this_cpu_dec(), given that preemption is disabled? > > That said, there isn't supposed to be any tracing during the now very > > small interval where RCU's idle-entry is incomplete. Mightn't it be > > better to (under CONFIG_PROVE_RCU or some such) give splats if tracing > > showed up in that interval? > > Again, tracing is not the issue. I do function tracing in that location > without any problems. The issue here was the stack tracer. > > Maybe we can create a new variable that is more cache local to the RCU > code. > > What about calling it "rcu_disabled"? Then tracing that depends on RCU > can simply check that. > > s/stack_trace_disable/disable_rcu/ > s/stack_trace_enable/enable_rcu/ > > export a per cpu variable rcu_disabled > > Then I can have the stack tracer check that variable as well. And we > could even put in a WARN_ON(this_cpu_read(rcu_disabled) in the > TRACE_EVENT() macros. > > Thoughts? At this point, if you can use the "__" versions, the change should be small. With that change, if no one else complains, I am OK. Thanx, Paul