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

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Date:   Fri, 6 Oct 2017 10:27:08 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v7 2/2] tracing: Add support for preempt and irq
 enable/disable events

Hi Steve,

On Fri, Oct 6, 2017 at 6:38 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 6 Oct 2017 00:28:21 -0700
> Joel Fernandes <joelaf@...gle.com> wrote:
>
>
>> Oh ok. So I traced this down to the original patch that added
>> time_hardirqs_off to lockdep. I *think* it was added just to keep the
>> irqsoff tracer working while lockdep is enabled, so that both lockdep
>> and the tracer would work at the same time. So I guess nobody noticed
>> any issues because nobody noticed or cared to check if irqsoff tracer
>> showed the same results with lockdep enabled vs disabled.
>
> As one of the main users of preemptirqsoff tracer, I can tell you that
> we didn't care ;-)

Hahah :-)

> Yes, it was added to keep them both working. It's dangerous to use
> CALLER_ADDR1, as lockdep can be called by the top frame, and ADDR1
> would go off the stack frame and cause a fault. I just put in ADDR0
> as filler as it required something to be added.

Ok. Then maybe I wrecked lockdep a bit in my patch from last night
that added tracepoint probes for it onto the new tracepoints [1].
Although it booted fine without any warnings. Considering your above
comment, do you see this an issue?

Initially I didn't mess around with lockdep but then Peter told me he
felt its weird PROVE_LOCKING makes the tracepoints unavailable. So
that led me to the probes patch (which includes all the lesson's
learnt from my v8 series [2]).

(Steve, btw I think the PATCH v8 series [2] can be a first step and a
safe solution for 4.15 while I work on the probe approach [1] but I
leave it to you how you want to time it :-)).

> We added a stack dump on new max latencies to figure out where the
> problem occurred.

Got it, that makes sense. thanks a lot for the history of it.

thanks,

- Joel

[1] https://lkml.org/lkml/2017/10/6/106
[2] https://lkml.org/lkml/2017/10/5/896

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