linux-kernel - Re: [PATCH v6 6/7] sched: add SD_BALANCE_NEWIDLE at MC and CPU level for sched

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Date:	Thu, 18 Dec 2008 15:05:38 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, suresh.b.siddha@...el.com,
	venkatesh.pallipadi@...el.com, a.p.zijlstra@...llo.nl,
	mingo@...e.hu, dipankar@...ibm.com, balbir@...ux.vnet.ibm.com,
	vatsa@...ux.vnet.ibm.com, ego@...ibm.com, andi@...stfloor.org,
	davecb@....com, tconnors@...ro.swin.edu.au, maxk@...lcomm.com,
	gregory.haskins@...il.com, pavel@...e.cz
Subject: Re: [PATCH v6 6/7] sched: add SD_BALANCE_NEWIDLE at MC and CPU
	level for sched_mc>0

* Andrew Morton <akpm@...ux-foundation.org> [2008-12-17 17:42:54]:

> On Wed, 17 Dec 2008 22:57:38 +0530
> Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:
> 
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -782,6 +782,16 @@ enum powersavings_balance_level {
> >  	((sched_mc_power_savings || sched_smt_power_savings) ?	\
> >  	 SD_POWERSAVINGS_BALANCE : 0)
> 
> What's with all the crappy macros in here?

Hi Andrew,

These macros set the SD_POWERSAVINGS_BALANCE flag based on the
sysfs tunable.
 
> > +/*
> > + * Optimise SD flags for power savings:
> > + * SD_BALANCE_NEWIDLE helps agressive task consolidation and power savings.
> > + * Keep default SD flags if sched_{smt,mc}_power_saving=0
> > + */
> > +
> > +#define POWERSAVING_SD_FLAGS	\
> > +	((sched_mc_power_savings || sched_smt_power_savings) ?	\
> > +	  SD_BALANCE_NEWIDLE : 0)
> 
> This one purports to be a constant, but it isn't - it's code.
> 
> It would be cleaner, clearer and more idiomatic to do
> 
> static inline int powersaving_sd_flags(void)
> {
> 	...
> }

Your are suggesting to move these to inline functions.   I will write
a patch and post for review.
 
> Also, doing (sched_mc_power_savings | sched_smt_power_saving) might
> save a branch.
> 
> >  #define test_sd_parent(sd, flag)	((sd->parent &&		\
> >  					 (sd->parent->flags & flag)) ? 1 : 0)
> 
> buggy when passed an expression with side-effects.  Doesn't need to be
> implemented as a macro.

Agreed, but these macros are used throughout sched.c and are
performance sensitive.  Inline functions are a close enough
replacement for the macro let me look for any performance penalty as
well and report.

Thanks for the review and comments.

--Vaidy
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