linux-kernel - Re: [BUG] idle_balance() does not call load_balance

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Message-ID: <20081209063448.GA17706@elte.hu>
Date:	Tue, 9 Dec 2008 07:34:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [BUG] idle_balance() does not call load_balance_newidle()


* Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:

> * Ingo Molnar <mingo@...e.hu> [2008-12-08 16:49:39]:
> 
> > 
> > * Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:
> > 
> > > Hi,
> > > 
> > > load_balance_newidle() does not get called if SD_BALANCE_NEWIDLE is
> > > set at higher level domain (3-CPU) and not in low level domain (2-MC).
> > > 
> > > pulled_task is initialised to -1 and checked for non-zero which is
> > > always true if the lowest level sched_domain does not have
> > > SD_BALANCE_NEWIDLE flag set.
> > > 
> > > Trivial fix to initialise pulled_task to zero.
> > > Patch against 2.6.28-rc7
> > 
> > applied to tip/sched/core, thanks! (Not for v2.6.28 because this could 
> > affect performance.)
> 
> Thanks Ingo.  This patch does not change any functionality in v2.6.28 
> and hence will not affect performance.  The SD flags are not touched. I 
> found this bug while setting different SD flags at MC level and CPU 
> level in my power saving balance patches.

if it does not change any functionality then we would not be doing the 
change, right?

It does change functionality, because:

> > > load_balance_newidle() does not get called if SD_BALANCE_NEWIDLE is 
> > > set at higher level domain (3-CPU) and not in low level domain 
> > > (2-MC).

even though it's a bug fix, it affects how the SD flags are interpreted 
and acted upon by the load balancer - i.e. the change can impact 
performance.

	Ingo
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