linux-kernel - Re: [PATCH] tracing/filters: add run-time field descriptions to TRACE_EVENT

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Date:	Wed, 1 Apr 2009 14:25:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] tracing/filters: add run-time field descriptions to
	TRACE_EVENT_FORMAT events


* Tom Zanussi <tzanussi@...il.com> wrote:

> > Oh, do you know why these unfiltered event are disappearing?
> 
> It's because of the way ring_buffer_discard() currently works - 
> the filtered events are still there in the ring buffer taking up 
> space; they just aren't visible in the output because the read 
> skips over them.
> 
> So if there's a high volume of events being recorded, any given 
> event can be quickly overwritten before it has a chance to be 
> read, or it may have been read and appeared in the output, but 
> between the time it was read and the next cat, could easily have 
> been overwritten, and therefore 'disappears' on the next cat.
> 
> It's really no different from the normal case where there is no 
> filtering in place - the same thing would happen but you wouldn't 
> notice whether a particular event was still there or not the next 
> time you cat'ed the file, because of the large volume of events 
> being displayed. It's just more apparent when most of the events 
> are discarded and invisible.
> 
> At least that's my theory based on my understanding of how the 
> ring-buffer works...

Correct, and this needs to be fixed.

	Ingo
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