linux-kernel - Re: [PATCH v2] tracing: Dump instance traces into dmesg on ftrace_dump_on

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Message-ID: <20221209125310.450aee97@gandalf.local.home>
Date:   Fri, 9 Dec 2022 12:53:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        mhiramat@...nel.org, Ross Zwisler <zwisler@...gle.com>
Subject: Re: [PATCH v2] tracing: Dump instance traces into dmesg on
 ftrace_dump_on_oops

On Thu, 10 Nov 2022 17:59:59 +0000
"Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:

> Currently ftrace only dumps the global trace buffer on an OOPs. For
> debugging a production usecase, I'd like to dump instance traces as
> well, into the kernel logs. The reason is we cannot use the global trace
> buffer as it may be used for other purposes.
> 
> This patch adds support for dumping the trace buffer instances along
> with the global trace buffer.
> 
> The instance traces are dumped first, and then the global trace buffer.

Sorry for the late review :-/

> 
> Cc: Ross Zwisler <zwisler@...gle.com>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/trace/trace.c | 67 ++++++++++++++++++++++++++------------------
>  1 file changed, 39 insertions(+), 28 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 47a44b055a1d..2cc75497d6d3 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9914,12 +9914,12 @@ trace_printk_seq(struct trace_seq *s)
>  	trace_seq_init(s);
>  }
>  
> -void trace_init_global_iter(struct trace_iterator *iter)
> +void trace_init_iter_with_tr(struct trace_iterator *iter, struct trace_array *tr)

Should be static. The trace_init_global_iter() is used by trace_kdb.c which
is why it wasn't static. Which means you also need to add that function
back, otherwise trace_kdb.c will not build.


>  {
> -	iter->tr = &global_trace;
> +	iter->tr = tr;
>  	iter->trace = iter->tr->current_trace;
>  	iter->cpu_file = RING_BUFFER_ALL_CPUS;
> -	iter->array_buffer = &global_trace.array_buffer;
> +	iter->array_buffer = &tr->array_buffer;
>  
>  	if (iter->trace && iter->trace->open)
>  		iter->trace->open(iter);
> @@ -9939,36 +9939,14 @@ void trace_init_global_iter(struct trace_iterator *iter)
>  	iter->fmt_size = STATIC_FMT_BUF_SIZE;
>  }
>  
> -void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
> +void ftrace_dump_one(struct trace_array *tr, enum ftrace_dump_mode oops_dump_mode)

Should also be static.

>  {
> -	/* use static because iter can be a bit big for the stack */
>  	static struct trace_iterator iter;
> -	static atomic_t dump_running;
> -	struct trace_array *tr = &global_trace;
>  	unsigned int old_userobj;
> -	unsigned long flags;
>  	int cnt = 0, cpu;
>  
> -	/* Only allow one dump user at a time. */
> -	if (atomic_inc_return(&dump_running) != 1) {
> -		atomic_dec(&dump_running);
> -		return;
> -	}
> -
> -	/*
> -	 * Always turn off tracing when we dump.
> -	 * We don't need to show trace output of what happens
> -	 * between multiple crashes.
> -	 *
> -	 * If the user does a sysrq-z, then they can re-enable
> -	 * tracing with echo 1 > tracing_on.
> -	 */
> -	tracing_off();

We need trace_tracing_off(tr) here.

> -
> -	local_irq_save(flags);
> -
>  	/* Simulate the iterator */
> -	trace_init_global_iter(&iter);
> +	trace_init_iter_with_tr(&iter, tr);
>  
>  	for_each_tracing_cpu(cpu) {
>  		atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
> @@ -9993,7 +9971,10 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>  		iter.cpu_file = RING_BUFFER_ALL_CPUS;
>  	}
>  
> -	printk(KERN_TRACE "Dumping ftrace buffer:\n");
> +	if (tr == &global_trace)
> +		printk(KERN_TRACE "Dumping ftrace buffer:\n");
> +	else
> +		printk(KERN_TRACE "Dumping ftrace instance %s buffer:\n", tr->name);
>  
>  	/* Did function tracer already get disabled? */
>  	if (ftrace_is_dead()) {
> @@ -10041,6 +10022,36 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>  	for_each_tracing_cpu(cpu) {
>  		atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
>  	}
> +}
> +
> +void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
> +{
> +	/* use static because iter can be a bit big for the stack */
> +	static atomic_t dump_running;
> +	struct trace_array *tr;
> +	unsigned long flags;
> +
> +	/* Only allow one dump user at a time. */
> +	if (atomic_inc_return(&dump_running) != 1) {
> +		atomic_dec(&dump_running);
> +		return;
> +	}
> +
> +	/*
> +	 * Always turn off tracing when we dump.
> +	 * We don't need to show trace output of what happens
> +	 * between multiple crashes.
> +	 *
> +	 * If the user does a sysrq-z, then they can re-enable
> +	 * tracing with echo 1 > tracing_on.
> +	 */
> +	tracing_off();
> +	local_irq_save(flags);
> +
> +	list_for_each_entry(tr, &ftrace_trace_arrays, list) {

I really do not want this to be the default. We should add a new option
that triggers this. Perhaps even specify the tracer to dump.

	ftrace_dump_on_oops=<tracer-name>

and

	echo <tracer-name> > /proc/sys/kernel/ftrace_dump_on_oops

But still have "echo 1" enable just the global_trace and "echo 0" disable
all.

-- Steve

> +		ftrace_dump_one(tr, oops_dump_mode);
> +	}
> +
>  	atomic_dec(&dump_running);
>  	local_irq_restore(flags);
>  }