From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934172AbdDGPIc (ORCPT ); Fri, 7 Apr 2017 11:08:32 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:45336 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934000AbdDGPIW (ORCPT ); Fri, 7 Apr 2017 11:08:22 -0400 Date: Fri, 7 Apr 2017 08:08:17 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton Subject: Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c Reply-To: paulmck@linux.vnet.ibm.com References: <20170407140106.051135969@goodmis.org> <20170407140308.502725512@goodmis.org> <20170407143619.GR1600@linux.vnet.ibm.com> <20170407104838.5de49e7b@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170407104838.5de49e7b@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17040715-0052-0000-0000-000001CC7678 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006893; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00844315; UDB=6.00416174; IPR=6.00622612; 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 15:08:19 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17040715-0053-0000-0000-00004FC3E5A8 Message-Id: <20170407150817.GU1600@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 10:48:38AM -0400, Steven Rostedt wrote: > On Fri, 7 Apr 2017 07:36:19 -0700 > "Paul E. McKenney" wrote: > > > On Fri, Apr 07, 2017 at 10:01:08AM -0400, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" > > > > > > The updates to the trace_active per cpu variable can be updated with the > > > this_cpu_*() functions as it only gets updated on the CPU that the variable > > > is on. > > > > > > Signed-off-by: Steven Rostedt (VMware) > > > --- > > > kernel/trace/trace_stack.c | 23 +++++++---------------- > > > 1 file changed, 7 insertions(+), 16 deletions(-) > > > > > > diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c > > > index 5fb1f2c87e6b..05ad2b86461e 100644 > > > --- a/kernel/trace/trace_stack.c > > > +++ b/kernel/trace/trace_stack.c > > > @@ -207,13 +207,12 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip, > > > struct ftrace_ops *op, struct pt_regs *pt_regs) > > > { > > > unsigned long stack; > > > - int cpu; > > > > > > preempt_disable_notrace(); > > > > > > - cpu = raw_smp_processor_id(); > > > /* no atomic needed, we only modify this variable by this cpu */ > > > - if (per_cpu(trace_active, cpu)++ != 0) > > > + this_cpu_inc(trace_active); > > > > For whatever it is worth... > > > > I was about to complain that this_cpu_inc() only disables preemption, > > not interrupts, but then I realized that any correct interrupt handler > > would have to restore the per-CPU variable to its original value. > > Yep, that's the reason for the comment about "no atomic needed". This > is a "stack modification". Any interruption in the flow will reset the > changes back to the way it was before going back to what it interrupted. > > > > > Presumably you have to sum up all the per-CPU trace_active counts, > > given that there is no guarantee that a process-level dec will happen > > on the same CPU that did the inc. > > That's why we disable preemption. We guarantee that a process-level dec > *will* happen on the same CPU that did the inc. But in that case, can't you use __this_cpu_inc()? Save a few cycles on RISC systems. Thanx, Paul > It's also the reason for the preemption disabled check in the > stack_tracer_disable() code. > > -- Steve > >