linux-kernel - Re: [PATCH 05/16] tracing: show that buffer size is not expanded

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Date:	Thu, 12 Mar 2009 23:20:45 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 05/16] tracing: show that buffer size is not expanded


On Fri, 13 Mar 2009, KOSAKI Motohiro wrote:

> > From: Steven Rostedt <srostedt@...hat.com>
> > 
> > Impact: do not confuse user on small trace buffer sizes
> > 
> > When the system boots up, the trace buffer is small to conserve memory.
> > It is only two pages per online CPU. When the tracer is used, it expands
> > to the default value.
> > 
> > This can confuse the user if they look at the buffer size and see only
> > 7, but then later they see 1408.
> > 
> >  # cat /debug/tracing/buffer_size_kb
> > 7
> > 
> >  # echo sched_switch > /debug/tracing/current_tracer
> > 
> >  # cat /debug/tracing/buffer_size_kb
> > 1408
> > 
> > This patch tries to help remove this confustion by showing that the
> > buffer has not been expanded.
> > 
> >  # cat /debug/tracing/buffer_size_kb
> > 7 (expanded: 1408)
> 
> Hi,
> 
> I have one question.
> Why souldn't use following output?
> 
> sprintf(buf, "%lu\n", trace_buf_size >> 10);
> 
> 
> My point is:
>   - pure number output can hadle easily.
>   - nobody need to know internal memory saving logic.

My answer to the second point is: "I do" ;-)

I like to know the real buffer size. That '7' comes from the ring buffer 
size directly. If something is going wrong, I do not want to hide the fact 
that the ring buffer size is not what I expect it to be. Lets say someone 
modifies the code, and we miss expanding the buffer. It will be very hard 
to debug why we are getting a small trace. But if we see that the buffer 
has not been expanded, we know exactly what is wrong.

-- Steve

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