linux-kernel - Re: [PATCH 1/2] tracing: Replace trace_event struct array with pointer array

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Date:	Wed, 02 Feb 2011 14:50:23 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rostedt@...dmis.org
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	akpm@...ux-foundation.org, fweisbec@...il.com,
	mathieu.desnoyers@...icios.com
Subject: Re: [PATCH 1/2] tracing: Replace trace_event struct array with
 pointer array

From: Steven Rostedt <rostedt@...dmis.org>
Date: Wed, 02 Feb 2011 13:06:14 -0500

> From: Steven Rostedt <srostedt@...hat.com>
> 
> Currently the trace_event structures are placed in the _ftrace_events
> section, and at link time, the linker makes one large array of all
> the trace_event structures. On boot up, this array is read (much like
> the initcall sections) and the events are processed.
> 
> The problem is that there is no guarantee that gcc will place complex
> structures nicely together in an array format. Two structures in the
> same file may be placed awkwardly, because gcc has no clue that they
> are suppose to be in an array.
> 
> A hack was used previous to force the alignment to 4, to pack the
> structures together. But this caused alignment issues with other
> architectures (sparc).
> 
> Instead of packing the structures into an array, the structures' addresses
> are now put into the _ftrace_event section. As pointers are always the
> natural alignment, gcc should always pack them tightly together
> (otherwise initcall, extable, etc would also fail).
> 
> By having the pointers to the structures in the section, we can still
> iterate the trace_events without causing unnecessary alignment problems
> with other architectures, or depending on the current behaviour of
> gcc that will likely change in the future just to tick us kernel developers
> off a little more.
> 
> The _ftrace_event section is also moved into the .init.data section
> as it is now only needed at boot up.
> 
> Suggested-by: David Miller <davem@...emloft.net>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by: David S. Miller <davem@...emloft.net>
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