linux-kernel - Re: [RFC PATCH v3 4/4] trace: Trace log handler for logging into STM blocks

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Date:	Wed, 8 Jul 2015 14:31:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chunyan Zhang <zhang.chunyan@...aro.org>
Cc:	rostedt@...dmis.org, mingo@...hat.com, mathieu.poirier@...aro.org,
	serge.broslavsky@...aro.org, broonie@...nel.org,
	alexander.shishkin@...ux.intel.com, zhang.lyra@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 4/4] trace: Trace log handler for logging into STM
 blocks

On Tue, Jul 07, 2015 at 06:10:43PM +0800, Chunyan Zhang wrote:
> Add the function 'trace_event_stm_output_##call' for printing events
> trace log into STM blocks.
> 
> This patch also adds a function call at where the events have been
> committed to ring buffer to export the trace event information to
> STM blocks.

So then you have two copies of the data, why that? Would a scheme were
data either goes to the STM or the regular buffer not make much more
sense?

> +++ b/include/trace/perf.h
> @@ -175,6 +175,7 @@ trace_event_raw_event_##call(void *__data, proto)			\
>  	{ assign; }							\
>  									\
>  	trace_event_buffer_commit(&fbuffer);				\
> +	trace_event_stm_log(&fbuffer);					\

This makes every trace event slower.

>  }
>  /*
>   * The ftrace_test_probe is compiled out, it is only here as a build time check
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