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 09:43:38 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <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

Hi Peter,

On Wed, Oct 4, 2017 at 9:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.

I meant to say the same thing (that I'm trying to ignore redundant
calls). In the last review you mentioned that it needs explaining, so
I added a comment. Should I just remove the comment then? Or I could
just state in a single line that the per-CPU variable is used for
ignoring redundant calls.

>> + */
>> +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?

Yes, I think that makes sense and reduces code duplication, I can do
this in the next rev.

thanks,

- Joel