Re: [PATCH 2/3] trace: fix default boot up tracer

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Feb 02, 2009 at 09:38:32PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.com>
> 
> Peter Zijlstra started the functionality to start up a default
> tracing at bootup. This patch finishes the work.
> 
> Now if you add 'ftrace=<tracer>' to the command line, when that tracer
> is registered on bootup, that tracer is selected and starts tracing.
> 
> Note, all selftests for tracers that are registered after this tracer
> is disabled. This prevents the selftests from disturbing the running
> tracer, or the running tracer from disturbing the selftest.
> 
> Signed-off-by: Steven Rostedt <srostedt@redhat.com>

Great!

> ---
>  kernel/trace/trace.c |   60 +++++++++++++++++++++++++++++++++++++++++++++-----
>  1 files changed, 54 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 2f8ac1f..2c720c7 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -53,6 +53,11 @@ unsigned long __read_mostly	tracing_thresh;
>   */
>  static bool __read_mostly tracing_selftest_running;
>  
> +/*
> + * If a tracer is running, we do not want to run SELFTEST.
> + */
> +static bool __read_mostly tracing_selftest_disabled;
> +
>  /* For tracers that don't implement custom flags */
>  static struct tracer_opt dummy_tracer_opt[] = {
>  	{ }
> @@ -110,14 +115,19 @@ static cpumask_var_t __read_mostly	tracing_buffer_mask;
>   */
>  int ftrace_dump_on_oops;
>  
> -static int tracing_set_tracer(char *buf);
> +static int tracing_set_tracer(const char *buf);
> +
> +#define BOOTUP_TRACER_SIZE		100
> +static char bootup_tracer_buf[BOOTUP_TRACER_SIZE] __initdata;
> +static char *default_bootup_tracer;
>  
>  static int __init set_ftrace(char *str)
>  {
> -	tracing_set_tracer(str);
> +	strncpy(bootup_tracer_buf, str, BOOTUP_TRACER_SIZE);
> +	default_bootup_tracer = bootup_tracer_buf;
>  	return 1;
>  }
> -__setup("ftrace", set_ftrace);
> +__setup("ftrace=", set_ftrace);
>  
>  static int __init set_ftrace_dump_on_oops(char *str)
>  {
> @@ -468,7 +478,7 @@ int register_tracer(struct tracer *type)
>  			type->flags->opts = dummy_tracer_opt;
>  
>  #ifdef CONFIG_FTRACE_STARTUP_TEST
> -	if (type->selftest) {
> +	if (type->selftest && !tracing_selftest_disabled) {
>  		struct tracer *saved_tracer = current_trace;
>  		struct trace_array *tr = &global_trace;
>  		int i;
> @@ -510,8 +520,25 @@ int register_tracer(struct tracer *type)
>   out:
>  	tracing_selftest_running = false;
>  	mutex_unlock(&trace_types_lock);
> -	lock_kernel();
>  
> +	if (!ret && default_bootup_tracer) {
> +		if (!strncmp(default_bootup_tracer, type->name,
> +			     BOOTUP_TRACER_SIZE)) {
> +			printk(KERN_INFO "Starting tracer '%s'\n",
> +			       type->name);
> +			/* Do we want this tracer to start on bootup? */
> +			tracing_set_tracer(type->name);
> +			default_bootup_tracer = NULL;
> +			/* disable other selftests, since this will break it. */
> +			tracing_selftest_disabled = 1;
> +#ifdef CONFIG_FTRACE_STARTUP_TEST
> +			printk(KERN_INFO "Disabling FTRACE selftests due"
> +			       " to running tracer '%s'\n", type->name);
> +#endif
> +		}
> +	}
> +
> +	lock_kernel();
>  	return ret;
>  }
>  
> @@ -2245,7 +2272,7 @@ tracing_set_trace_read(struct file *filp, char __user *ubuf,
>  	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
>  }
>  
> -static int tracing_set_tracer(char *buf)
> +static int tracing_set_tracer(const char *buf)
>  {
>  	struct trace_array *tr = &global_trace;
>  	struct tracer *t;
> @@ -3163,5 +3190,26 @@ out_free_buffer_mask:
>  out:
>  	return ret;
>  }
> +
> +__init static int clear_boot_tracer(void)
> +{
> +	/*
> +	 * The default tracer at boot buffer is an init section.
> +	 * This function is called in lateinit. If we did not
> +	 * find the boot tracer, then clear it out, to prevent
> +	 * later registration from accessing the buffer that is
> +	 * about to be freed.
> +	 */
> +	if (!default_bootup_tracer)
> +		return 0;
> +
> +	printk(KERN_INFO "ftrace bootup tracer '%s' not registered.\n",
> +	       default_bootup_tracer);
> +	default_bootup_tracer = NULL;
> +
> +	return 0;
> +}
> +
>  early_initcall(tracer_alloc_buffers);
>  fs_initcall(tracer_init_debugfs);
> +late_initcall(clear_boot_tracer);


That makes me think that perhaps a tracer would prefer to keep or not the tracing
when system_state looses the value SYSTEM_BOOTING.

Something similar to earlyprintk by adding a "keep" value in the ftrace parameter.

Perhaps it's better to wait for someone to request it...