linux-kernel - Re: [PATCH v7 4/8] sched: nominate preferred wakeup cpu

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Message-Id: <20081219135508.9c6217ba.akpm@linux-foundation.org>
Date:	Fri, 19 Dec 2008 13:55:08 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
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,
	svaidy@...ux.vnet.ibm.com, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH v7 4/8] sched: nominate preferred wakeup cpu

On Thu, 18 Dec 2008 23:26:22 +0530
Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:

> When the system utilisation is low and more cpus are idle,
> then the process waking up from sleep should prefer to
> wakeup an idle cpu from semi-idle cpu package (multi core
> package) rather than a completely idle cpu package which
> would waste power.
> 
> Use the sched_mc balance logic in find_busiest_group() to
> nominate a preferred wakeup cpu.
> 
> This info can be sored in appropriate sched_domain, but
> updating this info in all copies of sched_domain is not
> practical.  Hence this information is stored in root_domain
> struct which is one copy per partitioned sched domain.
> The root_domain can be accessed from each cpu's runqueue
> and there is one copy per partitioned sched domain.
> 

kernel/sched.c: In function 'find_busiest_group':
kernel/sched.c:3403: warning: passing argument 1 of '__first_cpu' from incompatible pointer type

Due to

	first_cpu(group_leader->cpumask);

apparently because Rusty changed sched_group.cpumask into a plain old
array and nobody tests their stuff against the tree into which it is
actually integrated :(

kernel/sched.c: In function 'schedule':
kernel/sched.c:3679: warning: 'active_balance' may be used uninitialized in this function

This warning is correct - the code is buggy.
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