linux-kernel - Re: [PATCH] dynamic debug: allow printing to trace event

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Message-ID: <20200723112644.7759f82f@oasis.local.home>
Date:   Thu, 23 Jul 2020 11:26:44 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        "jbaron@...mai.com" <jbaron@...mai.com>,
        "mingo@...hat.com" <mingo@...hat.com>, kernel <kernel@...s.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] dynamic debug: allow printing to trace event

On Thu, 23 Jul 2020 12:57:35 +0200
Vincent Whitchurch <vincent.whitchurch@...s.com> wrote:

> Would it be acceptable to just use a fixed size for the event?  At least
> for my own debugging use cases it's preferable to just have to increase
> the trace buffer size in case it's insufficient, rather than to have to
> restort to one-off debugging code.

There's two other options.

Option 1, is to allocate 256 bytes times 4 (in case of interruption,
where you have a separate buffer for every context - normal, softirq,
irq, nmi), and use it like I do for stack traces in the latest kernel
(see __ftrace_stack_trace() in kernel/trace/trace.c)

Option 2, would be to use trace_array_vprintk(), but you need to create
your own instance to do so to keep from messing with the top level buffer.

-- Steve