linux-kernel - Re: [PATCH] tracing: Add trace_trigger kernel command line option

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Date:   Thu, 20 Oct 2022 10:52:04 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [PATCH] tracing: Add trace_trigger kernel command line option

On Thu, Oct 20, 2022 at 10:43 AM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> Hi Steven,
>
> On Wed, Oct 19, 2022 at 8:01 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> >
> > Allow triggers to be enabled at kernel boot up. For example:
> >
> >   trace_trigger="sched_switch.stacktrace if prev_state == 2"
> >
> > The above will enable the stacktrace trigger on top of the sched_switch
> > event and only trigger if its prev_state is 2 (TASK_UNINTERRUPTIBLE). Then
> > at boot up, a stacktrace will trigger and be recorded in the tracing ring
> > buffer every time the sched_switch happens where the previous state is
> > TASK_INTERRUPTIBLE.
> >
> > As this calls into tracepoint logic during very early boot (before
> > interrupts are enabled), a check has to be done to see if early boot
> > interrupts are still disabled, and if so, avoid any call to RCU
> > synchronization, as that will enable interrupts and cause boot up issues.
> >
> > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> > ---
>
> rest_init() -> rcu_scheduler_starting() sets RCU_SCHEDULER_INIT
>
> start_kernel() disables interrupts and does the dance you mentioned,
> while setting that early_ flag. start_kernel() is called from arch
> code.
>
> From your crash, the trace trigger is happening before the early_ boot
> flag is set to true.

My apologies. I meant, before early_boot_irqs_disabled is set to false.