linux-kernel - [RFC][PATCH v2] tracing: Remove useless 4 bytes of padding from every event

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Date:	Wed, 16 May 2012 13:24:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	David Sharp <dhsharp@...gle.com>,
	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [RFC][PATCH v2] tracing: Remove useless 4 bytes of padding from
 every event

On Wed, 2012-05-16 at 09:44 -0400, Steven Rostedt wrote:
> Now that PowerTop v2 is out, which uses the parse-event library, it no
> longer is broken by the removal of the lock-depth field from every
> event. Currently we add 4 bytes of empty space to every event. If we
> have 1 million events, 4 million bytes are wasted in the ring buffers
> (for both ftrace and perf).
> 
> But this change will break PowerTop v1. Thus my question is, how long do
> we need to keep this wasted space in the ring buffers to satisfy an out
> of date tool?
> 

Would help if I compiled it :-)

Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 5f3f3be..f96dfef 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -49,7 +49,6 @@ struct trace_entry {
 	unsigned char		flags;
 	unsigned char		preempt_count;
 	int			pid;
-	int			padding;
 };
 
 #define FTRACE_MAX_EVENT						\

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 08a08ba..0960aa7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1145,7 +1145,6 @@ tracing_generic_entry_update(struct trace_entry *entry, unsigned long flags,
 
 	entry->preempt_count		= pc & 0xff;
 	entry->pid			= (tsk) ? tsk->pid : 0;
-	entry->padding			= 0;
 	entry->flags =
 #ifdef CONFIG_TRACE_IRQFLAGS_SUPPORT
 		(irqs_disabled_flags(flags) ? TRACE_FLAG_IRQS_OFF : 0) |
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 079a93a..5845731 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -116,7 +116,6 @@ static int trace_define_common_fields(void)
 	__common_field(unsigned char, flags);
 	__common_field(unsigned char, preempt_count);
 	__common_field(int, pid);
-	__common_field(int, padding);
 
 	return ret;
 }


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