linux-kernel - Re: [PATCH 7/6][RFC] sched: unify load_balance{,

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Date:	Thu, 24 Dec 2009 10:29:24 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	SureshSiddha <suresh.b.siddha@...el.com>,
	"Pallipadi,Venkatesh" <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH 7/6][RFC] sched: unify load_balance{,_newidle}()

On Thu, 2009-12-24 at 05:43 +0100, Mike Galbraith wrote:
> On Wed, 2009-12-23 at 16:13 +0100, Peter Zijlstra wrote:
> > load_balance() and load_balance_newidle() look remarkably similar, one
> > key point they differ in is the condition on when to active balance.
> > 
> > So split out that logic into a separate function.
> > 
> > One side effect is that previously load_balance_newidle() used to fail and
> > return -1 under these conditions, whereas now it doesn't. I've not yet fully
> > figured out the whole -1 return case for either load_balance{,_newidle}().
> > 
> > It also differs in that sd->cache_nice_tries is now added on the 
> > CPU_NEWLY_IDLE case.
> 
> Unification Looks like a good idea, less being more and all that.  I
> suspect that last bit is why newidle effectiveness has been heavily
> impacted.  x264 ultrafast testcase is whimpering pathetically again ;-)

That could be easily verified by setting cache_nice_tries to 0.

However, I would suspect need_active_balance(.idle = CPU_NEWLY_IDLE) to
always fail on your machine, since I don't think you've got all that
power savings muck enabled.

Is that with just this patch applied or also with the next one? I
worried more about the next one.

If just this one, that funny -1 return value thing might have played a
role, since that seems to trigger the:

 if (pulled_task) {
   this_rq->idle_stamp = 0;
   break;
 }

logic in idle_balance()

Which didn't make any sense to me, since it didn't move any task, so why
pretend it did...

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