Date: Wed, 1 Apr 2009 14:25:16 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Tom Zanussi <tzanussi@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] tracing/filters: add run-time field descriptions to
	TRACE_EVENT_FORMAT events
Message-ID: <20090401122516.GH12966@elte.hu>
References: <1238390524.6368.64.camel@bookworm> <20090330103808.GB6058@nowhere> <1238474638.10508.51.camel@bookworm>
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* Tom Zanussi <tzanussi@gmail.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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