linux-kernel - Re: [PATCH v12 0/3] tracing: Centralize preemptirq tracepoints and unify their usage

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Message-ID: <CAJWu+or50v6KYVESP3EZDa3i0NRVo7wXxvwqSGjoRxaGR3ZhgQ@mail.gmail.com>
Date:   Mon, 6 Aug 2018 08:24:19 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v12 0/3] tracing: Centralize preemptirq tracepoints and
 unify their usage

On Sun, Aug 5, 2018 at 7:07 PM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
> On Sun, 5 Aug 2018 09:46:56 -0700
> Joel Fernandes <joelaf@...gle.com> wrote:
>
>> Hi Masami,
>>
>> On Fri, Aug 3, 2018 at 9:51 PM, Joel Fernandes <joelaf@...gle.com> wrote:
>> [...]
>> >> On Thu, 2 Aug 2018 19:57:09 -0700
>> >> Joel Fernandes <joelaf@...gle.com> wrote:
>> >>
>> >>> Hi Masami,
>> >>>
>> >>> On Thu, Aug 2, 2018 at 7:55 AM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
>> >>> > Hi Joel,
>> >>> >
>> >>> > I found this caused several issues when testing ftrace.
>> >>> >
>> >>> > #1) ftrace boottest (FTRACE_STARTUP_TEST) fails
>> >>>
>> >>> This sadly appears to be a real issue. The startup test for
>> >>> "preemptirqsoff" tracer fails, however it passes for only preemptoff
>> >>> or only irqsoff. I tested only the last 2 tracers, not the first one,
>> >>> that's why I didn't catch it. I need to debug this more.
>>
>> I figured out this one too. Its because I need to account for
>> preempt_count() in tracer_hardirqs_off since the tracer probe is now
>> called with an additional level of preempt disabled from the
>> tracepoint code. Without that accounting, stop_critical_timings may
>> not be called causing an empty trace buffer. That should be easy to
>> fix, I'm on vacation though and back on 13th so can most likely look
>> at it only then (the week after the next).
>
> Nice! Thank you for identifying the root cause!

No problem :) Thanks a lot for your help!

>> >>> > #2) mmiotrace reports "IRQs not enabled as expected" error

I fixed #2 too now, the attached patch based on ftrace/core should fix
it (Sorry couldn't post the patch inline because I'm not at a sane
email client right now).

The patch is still in RFC shape, but I verified it fixes the issue.

Basically, I do this in the patch:
- switch to using SRCU for all tracepoints, use a separate SRCU node
for NMI context (Paul, Mathieu would love to get your thoughts here).
This is to remove dependence on sched-RCU.
- use tracepoints even after CPUs are offline

>> >>> > #3) lock subsystem event boottest causes "IRQs not disabled as expected" error (sometimes)
>> The only thing left to figure out is #3 ("lock subsystem event
>> boottest"). Could you let me know how to run this test?
>
> The #3 is not always reproducible, I just enabled these 2 configs
>
> CONFIG_FTRACE_STARTUP_TEST=y and CONFIG_EVENT_TRACE_TEST_SYSCALLS=y

Thanks, I am also able to see this now some of the times. I have
certain builds where this is much faster to reproduce so I'll use
those.

- Joel

View attachment "0001-tracepoint-Run-tracepoints-even-after-CPU-is-offline.patch" of type "text/x-patch" (6127 bytes)