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

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Date:   Thu, 5 Oct 2017 16:28:10 -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:
[...]
>> + */
>> +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?

In lockdep code, when trace_hardirqs_off is called,
trace_hardirqs_off_caller would pass CALLER_ADDR0 as
trace_hardirqs_off.

Because of this, the first argument passed to time_hardirqs_off would
always be an offset within trace_hardirqs_off:
time_hardirqs_off(CALLER_ADDR0, ip);

Is that intended? Seems to me that in the lockdep implementation of
trace_hardirqs_* in terms of *_caller(), we would completely miss the
second-last return address (CALLER_ADDR1) of trace_hardirqs_off().
Also for the above reasons, I don't think it doesn't make sense to use
this reuse logic for the tracer. Atleast I feel it might change the
current behavior of the preempt/irqsoff tracer which I don't intend to
change with my current patch set.

thanks,

- Joel