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    SubjectRe: [PATCH 01/15] tracing: Add a 'buffer_overwrite' debugfs file
    On Fri, Dec 3, 2010 at 5:57 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
    > 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".

    To do this, I had to add a special case to set_tracer_flags() to call
    ring_buffer_change_overwrite. It also means adding something not
    iterator-related to trace_iterator_flags. Although, maybe it's just
    the name of the enum and value prefix that should change.

    I'll send the new patch to you separately today.

    >
    > 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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