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

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Date:   Mon, 6 Aug 2018 11:07:29 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Joel Fernandes <joelaf@...gle.com>
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, 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!
BTW, I'm also on vacation this week :)

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

and boot on Qemu. I have seen this issue about 10% of system boot.

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>