linux-kernel - Re: [PATCH] ftrace: add a new 'tail drops' counter for overflow events

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Date:	Mon, 14 Mar 2011 22:39:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Slava Pestov <slavapestov@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mrubin@...gle.com, dhsharp@...gle.com
Subject: Re: [PATCH] ftrace: add a new 'tail drops' counter for overflow
 events

On Mon, 2011-03-14 at 15:53 -0700, Slava Pestov wrote:
> The existing 'overrun' counter is incremented when the ring
> buffer wraps around, with overflow on (the default). We wanted
> a way to count requests lost from the buffer filling up with
> overflow off, too. I decided to add a new counter instead
> of retro-fitting the existing one because it seems like a
> different statistic to count conceptually, and also because
> of how the code was structured.

So this is when we are in producer/consumer mode and the ring buffer
fills up and events are dropped.

For this we could just add a new ring buffer type. We could use the
RINGBUF_TYPE_TIME_STAMP as and call it RINGBUF_TYPE_LOST_EVENTS instead.
I never implemented the TIME_STAMP as I never found a need to ;)

As we currently have a TIME_EXTEND that is still relative from the last
event but has a total of 59 bits for time. That being nanoseconds we can
handle events that are 18 years apart. That far apart and never being
read.

The LOST_EVENTS could store the number of events lost when it starts
reading again. This way raw readers will know that events were lost and
how many.

-- Steve

> 
> Signed-Off-By: Slava Pestov <slavapestov@...gle.com>

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