linux-kernel - Re: [PATCH 2/2] sched: make sched_param arugment static variables in some sched

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Date:	Tue, 6 Jul 2010 16:12:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	rostedt@...dmis.org
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Minchan Kim <minchan.kim@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	James Morris <jmorris@...ei.org>
Subject: Re: [PATCH 2/2] sched: make sched_param arugment static variables
 in some sched_setscheduler() caller

On Tue, 06 Jul 2010 18:13:58 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 2010-07-06 at 09:51 +0900, KOSAKI Motohiro wrote:
> > Andrew Morton pointed out almost sched_setscheduler() caller are
> > using fixed parameter and it can be converted static. it reduce
> > runtume memory waste a bit.
> 
> We are replacing runtime waste with permanent waste?

Confused.  kernel/trace/ appears to waste resources by design, so what's
the issue?

I don't think this change will cause more waste.  It'll consume 4 bytes
of .data and will save a little more .text.

> > 
> > Reported-by: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> 
> 
> 
> > --- a/kernel/trace/trace_selftest.c
> > +++ b/kernel/trace/trace_selftest.c
> > @@ -560,7 +560,7 @@ trace_selftest_startup_nop(struct tracer *trace, struct trace_array *tr)
> >  static int trace_wakeup_test_thread(void *data)
> >  {
> >  	/* Make this a RT thread, doesn't need to be too high */
> > -	struct sched_param param = { .sched_priority = 5 };
> > +	static struct sched_param param = { .sched_priority = 5 };
> >  	struct completion *x = data;
> >  
> 
> This is a thread that runs on boot up to test the sched_wakeup tracer.
> Then it is deleted and all memory is reclaimed.
> 
> Thus, this patch just took memory that was usable at run time and
> removed it permanently.
> 
> Please Cc me on all tracing changes.

Well if we're so worried about resource wastage then how about making
all boot-time-only text and data reside in __init and __initdata
sections rather than hanging around uselessly in memory for ever?

Only that's going to be hard because we went and added pointers into
.init.text from .data due to `struct tracer.selftest', which will cause
a storm of section mismatch warnings.  Doh, should have invoked the
selftests from initcalls.  That might open the opportunity of running
the selftests by modprobing the selftest module, too.

And I _do_ wish the selftest module was modprobeable, rather than this
monstrosity:

#ifdef CONFIG_FTRACE_SELFTEST
/* Let selftest have access to static functions in this file */
#include "trace_selftest.c"
#endif

Really?  Who had a tastebudectomy over there?  At least call it
trace_selftest.inc or something, so poor schmucks don't go scrabbling
around wondering "how the hell does this thing get built oh no they
didn't really go and #include it did they?"

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