linux-kernel - Re: [PATCH 03/32] ring-buffer: Add interface for setting absolute time stamps

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Date:   Fri, 14 Jul 2017 11:14:46 -0500
From:   Tom Zanussi <tom.zanussi@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        vedang.patel@...el.com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH 03/32] ring-buffer: Add interface for setting absolute
 time stamps

Hi Namhyung,

On Fri, 2017-07-14 at 14:25 +0900, Namhyung Kim wrote:
> On Mon, Jun 26, 2017 at 05:49:04PM -0500, Tom Zanussi wrote:
> > Define a new function, tracing_set_time_stamp_abs(), which can be used
> > to enable or disable the use of absolute timestamps rather than time
> > deltas for a trace array.
> > 
> > This resets the buffer to prevent a mix of time deltas and absolute
> > timestamps.
> > 
> > Only the interface is added here; a subsequent patch will add the
> > underlying implementation.
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > ---
> >  include/linux/ring_buffer.h |  2 ++
> >  kernel/trace/ring_buffer.c  | 11 +++++++++++
> >  kernel/trace/trace.c        | 25 ++++++++++++++++++++++++-
> >  kernel/trace/trace.h        |  2 ++
> >  4 files changed, 39 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> > index ee9b461..28e3472 100644
> > --- a/include/linux/ring_buffer.h
> > +++ b/include/linux/ring_buffer.h
> > @@ -180,6 +180,8 @@ void ring_buffer_normalize_time_stamp(struct ring_buffer *buffer,
> >  				      int cpu, u64 *ts);
> >  void ring_buffer_set_clock(struct ring_buffer *buffer,
> >  			   u64 (*clock)(void));
> > +void ring_buffer_set_time_stamp_abs(struct ring_buffer *buffer, bool abs);
> > +bool ring_buffer_time_stamp_abs(struct ring_buffer *buffer);
> >  
> >  size_t ring_buffer_page_len(void *page);
> >  
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 4ae268e..f544738 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -485,6 +485,7 @@ struct ring_buffer {
> >  	u64				(*clock)(void);
> >  
> >  	struct rb_irq_work		irq_work;
> > +	bool				time_stamp_abs;
> >  };
> >  
> >  struct ring_buffer_iter {
> > @@ -1379,6 +1380,16 @@ void ring_buffer_set_clock(struct ring_buffer *buffer,
> >  	buffer->clock = clock;
> >  }
> >  
> > +void ring_buffer_set_time_stamp_abs(struct ring_buffer *buffer, bool abs)
> > +{
> > +	buffer->time_stamp_abs = abs;
> > +}
> > +
> > +bool ring_buffer_time_stamp_abs(struct ring_buffer *buffer)
> > +{
> > +	return buffer->time_stamp_abs;
> > +}
> > +
> >  static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
> >  
> >  static inline unsigned long rb_page_entries(struct buffer_page *bpage)
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 19ac208..da8cd51 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -2169,7 +2169,7 @@ struct ring_buffer_event *
> >  
> >  	*current_rb = trace_file->tr->trace_buffer.buffer;
> >  
> > -	if ((trace_file->flags &
> > +	if (!ring_buffer_time_stamp_abs(*current_rb) && (trace_file->flags &
> >  	     (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
> >  	    (entry = this_cpu_read(trace_buffered_event))) {
> >  		/* Try to use the per cpu buffer first */
> > @@ -6079,6 +6079,29 @@ static int tracing_clock_open(struct inode *inode, struct file *file)
> >  	return ret;
> >  }
> >  
> > +int tracing_set_time_stamp_abs(struct trace_array *tr, bool abs)
> > +{
> > +	mutex_lock(&trace_types_lock);
> > +
> > +	ring_buffer_set_time_stamp_abs(tr->trace_buffer.buffer, abs);
> > +
> > +	/*
> > +	 * New timestamps may not be consistent with the previous setting.
> > +	 * Reset the buffer so that it doesn't have incomparable timestamps.
> > +	 */
> > +	tracing_reset_online_cpus(&tr->trace_buffer);
> > +
> > +#ifdef CONFIG_TRACER_MAX_TRACE
> > +	if (tr->flags & TRACE_ARRAY_FL_GLOBAL && tr->max_buffer.buffer)
> > +		ring_buffer_set_time_stamp_abs(tr->max_buffer.buffer, abs);
> > +	tracing_reset_online_cpus(&tr->max_buffer);
> 
> Why do you do this only for the global array?  AFAIK instance arrays
> can have the max buffer too.  Am I missing something?
> 

Yes, I think you're right.  Will take a closer look and change as
needed.

Thanks,

Tom