linux-kernel - Re: [PATCH v2 08/40] ring-buffer: Redefine the unimplemented RINGBUF_TIME_TIME

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Message-ID: <20170907103510.6626368d@gandalf.local.home>
Date:   Thu, 7 Sep 2017 10:35:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     tglx@...utronix.de, mhiramat@...nel.org, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel.opensrc@...il.com, joelaf@...gle.com,
        mathieu.desnoyers@...icios.com, baohong.liu@...el.com,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 08/40] ring-buffer: Redefine the unimplemented
 RINGBUF_TIME_TIME_STAMP

On Tue,  5 Sep 2017 16:57:20 -0500
Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index 28e3472..74bc276 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -36,10 +36,12 @@ struct ring_buffer_event {
>   *				 array[0] = time delta (28 .. 59)
>   *				 size = 8 bytes
>   *
> - * @RINGBUF_TYPE_TIME_STAMP:	Sync time stamp with external clock
> - *				 array[0]    = tv_nsec
> - *				 array[1..2] = tv_sec
> - *				 size = 16 bytes
> + * @RINGBUF_TYPE_TIME_STAMP:	Absolute timestamp
> + *				 Same format as TIME_EXTEND except that the
> + *				 value is an absolute timestamp, not a delta
> + *				 event.time_delta contains bottom 27 bits
> + *				 array[0] = top (28 .. 59) bits
> + *				 size = 8 bytes

Is it going to be an issue that our time stamp is only 59 bits?

2^59 = 576,460,752,303,423,488

Thus, 2^59 nanoseconds (I doubt we will need to have precision better
than nanoseconds) = 576,460,752 seconds = 9,607,679 minutes = 160,127
hours = 6,671 days = 18 years.

We would be screwed if we trace for more than 18 years. ;-)

That's why I had it as 16 bytes, to be able to hold a full 64 bit
timestamp (and still be 8 byte aligned). But since we've gone this long
without needing this, I'm sure a 59 bit absolute timestamp should be
good enough.

>   *
>   * <= @RINGBUF_TYPE_DATA_TYPE_LEN_MAX:
>   *				Data record
> @@ -56,12 +58,12 @@ enum ring_buffer_type {
>  	RINGBUF_TYPE_DATA_TYPE_LEN_MAX = 28,
>  	RINGBUF_TYPE_PADDING,
>  	RINGBUF_TYPE_TIME_EXTEND,
> -	/* FIXME: RINGBUF_TYPE_TIME_STAMP not implemented */
>  	RINGBUF_TYPE_TIME_STAMP,
>  };
>  
>  unsigned ring_buffer_event_length(struct ring_buffer_event *event);
>  void *ring_buffer_event_data(struct ring_buffer_event *event);
> +u64 ring_buffer_event_time_stamp(struct ring_buffer_event *event);
>  
>  /*
>   * ring_buffer_discard_commit will remove an event that has not




> @@ -2488,6 +2519,10 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
>  {
>  	u64 delta;
>  
> +	/* In TIME_STAMP mode, write_stamp is unused, nothing to do */

No, we still need to keep the write_stamp updated. Sure, it doesn't use
it, but I do want to have absolute and delta timestamps working
together in a single buffer. It shouldn't be one or the other. In fact,
I plan on using it that way for nested events.

Maybe for this feature we can let it slide. But I will be working on
fixing that.

-- Steve

> +	if (event->type_len == RINGBUF_TYPE_TIME_STAMP)
> +		return;
> +
>  	/*
>  	 * The event first in the commit queue updates the
>  	 * time stamp.
> @@ -2501,9 +2536,7 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
>  			cpu_buffer->write_stamp =
>  				cpu_buffer->commit_page->page->time_stamp;
>  		else if (event->type_len == RINGBUF_TYPE_TIME_EXTEND) {
> -			delta = event->array[0];
> -			delta <<= TS_SHIFT;
> -			delta += event->time_delta;
> +			delta = ring_buffer_event_time_stamp(event);
>  			cpu_buffer->write_stamp += delta;
>  		} else