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From:   Ross Zwisler <zwisler@google.com>
Date:   Thu, 8 Dec 2022 15:27:07 -0700
Message-ID: <CAGRrVHw2ABuBtb+6BtES0WPNpbtu3p6vfZ-pADjtYYxm9kWZ4g@mail.gmail.com>
Subject: Re: [PATCH v2] tracing: Add trace_trigger kernel command line option
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        paulmck@kernel.org, Joel Fernandes <joel@joelfernandes.org>,
        Tom Zanussi <zanussi@kernel.org>
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On Thu, Oct 20, 2022 at 7:00 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> From: "Steven Rostedt (Google)" <rostedt@goodmis.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.
>
> Another useful trigger would be "traceoff" which can stop tracing on an
> event if a field of the event matches a certain value defined by the
> filter ("if" statement).
>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

<>

> +#ifdef CONFIG_HIST_TRIGGERS

Can you help me understand why this is only available if
CONFIG_HIST_TRIGGERS is selected in the kernel config?  AFAICT this
code doesn't depend on the histogram code, and the run-time selection
of triggers is usable without CONFIG_HIST_TRIGGERS.

Thanks!

> +static void trace_early_triggers(struct trace_event_file *file, const char *name)
> +{
> +       int ret;
> +       int i;

<>