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 16:29:40 -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

On Thu, Oct 5, 2017 at 4:28 PM, Joel Fernandes <joelaf@...gle.com> wrote:
> 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:
[...]
>>>  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

Sorry, I got lost with double-negatives here, I meant "I don't think
it makes sense to.."

thanks,

- Joel