From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756341AbdDGOg3 (ORCPT ); Fri, 7 Apr 2017 10:36:29 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:44044 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756330AbdDGOgX (ORCPT ); Fri, 7 Apr 2017 10:36:23 -0400 Date: Fri, 7 Apr 2017 07:36:19 -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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170407140308.502725512@goodmis.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17040714-0008-0000-0000-000001EA6F65 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006893; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00844304; UDB=6.00416168; IPR=6.00622601; 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 14:36:20 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17040714-0009-0000-0000-00003475B805 Message-Id: <20170407143619.GR1600@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-1704070122 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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. Thanx, Paul > + if (this_cpu_read(trace_active) != 1) > goto out; > > ip += MCOUNT_INSN_SIZE; > @@ -221,7 +220,7 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip, > check_stack(ip, &stack); > > out: > - per_cpu(trace_active, cpu)--; > + this_cpu_dec(trace_active); > /* prevent recursion in schedule */ > preempt_enable_notrace(); > } > @@ -253,7 +252,6 @@ stack_max_size_write(struct file *filp, const char __user *ubuf, > long *ptr = filp->private_data; > unsigned long val, flags; > int ret; > - int cpu; > > ret = kstrtoul_from_user(ubuf, count, 10, &val); > if (ret) > @@ -266,14 +264,13 @@ stack_max_size_write(struct file *filp, const char __user *ubuf, > * we will cause circular lock, so we also need to increase > * the percpu trace_active here. > */ > - cpu = smp_processor_id(); > - per_cpu(trace_active, cpu)++; > + this_cpu_inc(trace_active); > > arch_spin_lock(&stack_trace_max_lock); > *ptr = val; > arch_spin_unlock(&stack_trace_max_lock); > > - per_cpu(trace_active, cpu)--; > + this_cpu_dec(trace_active); > local_irq_restore(flags); > > return count; > @@ -307,12 +304,9 @@ t_next(struct seq_file *m, void *v, loff_t *pos) > > static void *t_start(struct seq_file *m, loff_t *pos) > { > - int cpu; > - > local_irq_disable(); > > - cpu = smp_processor_id(); > - per_cpu(trace_active, cpu)++; > + this_cpu_inc(trace_active); > > arch_spin_lock(&stack_trace_max_lock); > > @@ -324,12 +318,9 @@ static void *t_start(struct seq_file *m, loff_t *pos) > > static void t_stop(struct seq_file *m, void *p) > { > - int cpu; > - > arch_spin_unlock(&stack_trace_max_lock); > > - cpu = smp_processor_id(); > - per_cpu(trace_active, cpu)--; > + this_cpu_dec(trace_active); > > local_irq_enable(); > } > -- > 2.10.2 > >