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[PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c

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  [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c Steven Rostedt <rostedt@goodmis.org> - 2017-04-07 16:10 +0200
    Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in  trace_stack.c "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-07 16:40 +0200
      Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu()  in trace_stack.c Steven Rostedt <rostedt@goodmis.org> - 2017-04-07 17:00 +0200
        Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in  trace_stack.c "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-07 17:20 +0200

#1618819 — [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

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) <rostedt@goodmis.org>
---
 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);
+	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

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#1618853 — Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c

On Fri, Apr 07, 2017 at 10:01:08AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> 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) <rostedt@goodmis.org>
> ---
>  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
> 
> 

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#1618877 — Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c

On Fri, 7 Apr 2017 07:36:19 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Fri, Apr 07, 2017 at 10:01:08AM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > 
> > 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) <rostedt@goodmis.org>
> > ---
> >  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.

It's also the reason for the preemption disabled check in the
stack_tracer_disable() code.

-- Steve

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#1618888 — Re: [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c

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" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > On Fri, Apr 07, 2017 at 10:01:08AM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > 
> > > 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) <rostedt@goodmis.org>
> > > ---
> > >  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
> 
> 

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