Subject: Re: [PATCH 01/15] tracing: Add a 'buffer_overwrite' debugfs file
From: Steven Rostedt <rostedt@goodmis.org>
To: David Sharp <dhsharp@google.com>
Cc: linux-kernel@vger.kernel.org, mrubin@google.com,
        Jiaying Zhang <jiayingz@google.com>
In-Reply-To: <1291421609-14665-2-git-send-email-dhsharp@google.com>
References: <1291421609-14665-1-git-send-email-dhsharp@google.com>
	 <1291421609-14665-2-git-send-email-dhsharp@google.com>
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Date: Fri, 03 Dec 2010 20:57:10 -0500
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On Fri, 2010-12-03 at 16:13 -0800, David Sharp wrote:
> From: Jiaying Zhang <jiayingz@google.com>
> 
> Add a "buffer_overwrite" debugfs file for ftrace to control whether the buffer

Instead of adding a new file, make this another "trace_option".

Thanks,

-- Steve


> should be overwritten on overflow or not. The default remains to overwrite old
> events when the buffer is full. This patch adds the option to instead discard
> newest events when the buffer is full. This is useful to get a snapshot of
> traces just after enabling traces. Dropping the current event is also a simpler
> code path.
> 
> Signed-off-by: David Sharp <dhsharp@google.com>
> ---
>  Documentation/trace/ftrace.txt |    6 ++++
>  include/linux/ring_buffer.h    |    2 +
>  kernel/trace/ring_buffer.c     |   11 +++++++
>  kernel/trace/trace.c           |   59 +++++++++++++++++++++++++++++++++++++---
>  4 files changed, 74 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
> index 557c1ed..9237da3 100644
> --- a/Documentation/trace/ftrace.txt
> +++ b/Documentation/trace/ftrace.txt
> @@ -138,6 +138,12 @@ of ftrace. Here is a list of some of the key files:
>  	This can only be updated when the current_tracer
>  	is set to "nop".
>  
> +  buffer_overwrite:
> +
> +	This controls what happens when the trace buffer is full.
> +	If "1" (default), the oldest events are discarded and 
> +	overwritten. If "0", then the newest events are discarded.
> +
>    tracing_cpumask:
>  
>  	This is a mask that lets the user only trace
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index 8d3a248..ab38ac8 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -100,6 +100,8 @@ void ring_buffer_free(struct ring_buffer *buffer);
>  
>  int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size);
>  
> +void ring_buffer_change_overwrite(struct ring_buffer *buffer, int val);
> +
>  struct ring_buffer_event *ring_buffer_lock_reserve(struct ring_buffer *buffer,
>  						   unsigned long length);
>  int ring_buffer_unlock_commit(struct ring_buffer *buffer,
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 9ed509a..3207147 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1429,6 +1429,17 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_resize);
>  
> +void ring_buffer_change_overwrite(struct ring_buffer *buffer, int val)
> +{
> +	mutex_lock(&buffer->mutex);
> +	if (val)
> +		buffer->flags |= RB_FL_OVERWRITE;
> +	else
> +		buffer->flags &= ~RB_FL_OVERWRITE;
> +	mutex_unlock(&buffer->mutex);
> +}
> +EXPORT_SYMBOL_GPL(ring_buffer_change_overwrite);
> +
>  static inline void *
>  __rb_data_page_index(struct buffer_data_page *bpage, unsigned index)
>  {
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c380612..ed5c14f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -41,8 +41,6 @@
>  #include "trace.h"
>  #include "trace_output.h"
>  
> -#define TRACE_BUFFER_FLAGS	(RB_FL_OVERWRITE)
> -
>  /*
>   * On boot up, the ring buffer is set to the minimum size, so that
>   * we do not waste memory on systems that are not using tracing.
> @@ -241,6 +239,9 @@ int tracing_is_enabled(void)
>  
>  static unsigned long		trace_buf_size = TRACE_BUF_SIZE_DEFAULT;
>  
> +/* whether the trace buffer should be overwritten on overflow or not. */
> +static enum ring_buffer_flags  trace_buffer_flags = RB_FL_OVERWRITE;
> +
>  /* trace_types holds a link list of available tracers. */
>  static struct tracer		*trace_types __read_mostly;
>  
> @@ -3466,6 +3467,47 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
>  	return cnt;
>  }
>  
> +static ssize_t
> +tracing_overwrite_read(struct file *filp, char __user *ubuf,
> +		     size_t cnt, loff_t *ppos)
> +{
> +	char buf[64];
> +	int r;
> +	r = snprintf(buf, 64, "%u\n", trace_buffer_flags);
> +	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
> +}
> +
> +static ssize_t
> +tracing_overwrite_write(struct file *filp, const char __user *ubuf,
> +		      size_t cnt, loff_t *ppos)
> +{
> +	unsigned long val;
> +	char buf[64];
> +	int ret;
> +
> +	if (cnt >= sizeof(buf))
> +		return -EINVAL;
> +
> +	if (copy_from_user(&buf, ubuf, cnt))
> +		return -EFAULT;
> +
> +	buf[cnt] = 0;
> +
> +	ret = strict_strtoul(buf, 10, &val);
> +	if (ret < 0)
> +		return ret;
> +	*ppos += cnt;
> +
> +	if (val != 0 && val != 1)
> +		return -EINVAL;
> +
> +	if (trace_buffer_flags != val) {
> +		trace_buffer_flags = val;
> +		ring_buffer_change_overwrite(global_trace.buffer, val);
> +	}
> +	return cnt;
> +}
> +
>  static int mark_printk(const char *fmt, ...)
>  {
>  	int ret;
> @@ -3611,6 +3653,12 @@ static const struct file_operations tracing_entries_fops = {
>  	.llseek		= generic_file_llseek,
>  };
>  
> +static const struct file_operations tracing_overwrite_fops = {
> +	.open		= tracing_open_generic,
> +	.read		= tracing_overwrite_read,
> +	.write		= tracing_overwrite_write,
> +};
> +
>  static const struct file_operations tracing_mark_fops = {
>  	.open		= tracing_open_generic,
>  	.write		= tracing_mark_write,
> @@ -4336,6 +4384,9 @@ static __init int tracer_init_debugfs(void)
>  	trace_create_file("buffer_size_kb", 0644, d_tracer,
>  			&global_trace, &tracing_entries_fops);
>  
> +	trace_create_file("buffer_overwrite", 0644, d_tracer,
> +			&global_trace, &tracing_overwrite_fops);
> +
>  	trace_create_file("trace_marker", 0220, d_tracer,
>  			NULL, &tracing_mark_fops);
>  
> @@ -4565,7 +4616,7 @@ __init static int tracer_alloc_buffers(void)
>  
>  	/* TODO: make the number of buffers hot pluggable with CPUS */
>  	global_trace.buffer = ring_buffer_alloc(ring_buf_size,
> -						   TRACE_BUFFER_FLAGS);
> +						   trace_buffer_flags);
>  	if (!global_trace.buffer) {
>  		printk(KERN_ERR "tracer: failed to allocate ring buffer!\n");
>  		WARN_ON(1);
> @@ -4575,7 +4626,7 @@ __init static int tracer_alloc_buffers(void)
>  
> 
>  #ifdef CONFIG_TRACER_MAX_TRACE
> -	max_tr.buffer = ring_buffer_alloc(1, TRACE_BUFFER_FLAGS);
> +	max_tr.buffer = ring_buffer_alloc(1, trace_buffer_flags);
>  	if (!max_tr.buffer) {
>  		printk(KERN_ERR "tracer: failed to allocate max ring buffer!\n");
>  		WARN_ON(1);


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