linux-kernel - Re: [PATCH v7 2/2] tracing: Add support for preempt and irq enable/disable events

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Date:   Wed, 4 Oct 2017 18:01:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v7 2/2] tracing: Add support for preempt and irq
 enable/disable events

On Fri, Sep 29, 2017 at 02:22:45PM -0700, Joel Fernandes wrote:
> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
> index 0e3033c00474..515ac851841a 100644
> --- a/kernel/trace/trace_irqsoff.c
> +++ b/kernel/trace/trace_irqsoff.c
> @@ -16,6 +16,9 @@
>  
>  #include "trace.h"
>  
> +#define CREATE_TRACE_POINTS
> +#include <trace/events/preemptirq.h>
> +
>  #if defined(CONFIG_IRQSOFF_TRACER) || defined(CONFIG_PREEMPT_TRACER)
>  static struct trace_array		*irqsoff_trace __read_mostly;
>  static int				tracer_enabled __read_mostly;
> @@ -776,27 +779,60 @@ static inline void tracer_preempt_on(unsigned long a0, unsigned long a1) { }
>  static inline void tracer_preempt_off(unsigned long a0, unsigned long a1) { }
>  #endif
>  
> +/*
> + * trace_hardirqs_off can be called even when IRQs are already off.

In fact it must be.. otherwise you'll get a complaint.

>     It is
> + * pointless and inconsistent with trace_preempt_enable and
> + * trace_preempt_disable to trace this, lets prevent double counting it with a
> + * per-cpu variable. Also reuse the per-cpu variable for other trace_hardirqs_*
> + * functions since we already define it.

Lockdep ignores redundant calls. But I'm not entirely sure what the
above is trying to say.

> + */
> +static DEFINE_PER_CPU(int, tracing_irq_cpu);
> +
>  #if defined(CONFIG_TRACE_IRQFLAGS) && !defined(CONFIG_PROVE_LOCKING)
>  void trace_hardirqs_on(void)
>  {
> +	if (!this_cpu_read(tracing_irq_cpu))
> +		return;
> +
> +	trace_irq_enable_rcuidle(CALLER_ADDR0, CALLER_ADDR1);
>  	tracer_hardirqs_on();
> +
> +	this_cpu_write(tracing_irq_cpu, 0);
>  }
>  EXPORT_SYMBOL(trace_hardirqs_on);
>  
>  void trace_hardirqs_off(void)
>  {
> +	if (this_cpu_read(tracing_irq_cpu))
> +		return;
> +
> +	this_cpu_write(tracing_irq_cpu, 1);
> +
> +	trace_irq_disable_rcuidle(CALLER_ADDR0, CALLER_ADDR1);
>  	tracer_hardirqs_off();
>  }
>  EXPORT_SYMBOL(trace_hardirqs_off);
>  
>  __visible void trace_hardirqs_on_caller(unsigned long caller_addr)
>  {
> +	if (!this_cpu_read(tracing_irq_cpu))
> +		return;
> +
> +	trace_irq_enable_rcuidle(CALLER_ADDR0, caller_addr);
>  	tracer_hardirqs_on_caller(caller_addr);
> +
> +	this_cpu_write(tracing_irq_cpu, 0);
>  }
>  EXPORT_SYMBOL(trace_hardirqs_on_caller);
>  
>  __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
>  {
> +	if (this_cpu_read(tracing_irq_cpu))
> +		return;
> +
> +	this_cpu_write(tracing_irq_cpu, 1);
> +
> +	trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
>  	tracer_hardirqs_off_caller(caller_addr);
>  }
>  EXPORT_SYMBOL(trace_hardirqs_off_caller);

lockdep implements the trace_hardirq_*() in terms of *_caller(). Would
that make sense here?