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

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Date:	Sat, 20 Dec 2008 10:06:38 +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,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH v7 4/8] sched: nominate preferred wakeup cpu

* Andrew Morton <akpm@...ux-foundation.org> [2008-12-19 13:55:08]:

> 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 :(

Hi Andrew,

I agree.  These are integration issues and I will test better next time.

There were two such bugs which Ingo fixed in the tip.

commit 220e7f617826e0527bbc523ba859f6a4bae0bfe1
Author: Ingo Molnar <mingo@...e.hu>
Date:   Fri Dec 19 00:53:40 2008 +0100

    sched: fix warning in kernel/sched.c
    
    Impact: fix cpumask conversion bug
    
    this warning:
    
      kernel/sched.c: In function find_busiest_group:
      kernel/sched.c:3429: warning: passing argument 1 of __first_cpu from incompatible pointer type
    
    shows that we forgot to convert a new patch to the new cpumask APIs.
    
    Signed-off-by: Ingo Molnar <mingo@...e.hu>


commit edb4c71953409c1deac1a80528ac0aa768762b33
Author: Ingo Molnar <mingo@...e.hu>
Date:   Thu Dec 18 21:30:23 2008 +0100

    sched: move test_sd_parent() to an SMP section of sched.h
    
    Impact: build fix
    
    Signed-off-by: Ingo Molnar <mingo@...e.hu>

I have tested the sched-tip with these fixes.

> 
> 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.

Yes this is my code bug.  I did not see the warning in sched.c.  Is
there any build option that I need to pass in order to get -Wall
effect?

Here is the fix to initialise the active_balance=0.

Thanks for the detailed review.  I will work to improve the quality of
my submission.

--Vaidy

    sched: bug fix -- initialise active_balance variable
    
    In sched.c load_balance_newidle, potential use of uninitialised
    variable.
    
    Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>

diff --git a/kernel/sched.c b/kernel/sched.c
index e6a88bf..a21fe6d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3692,7 +3692,7 @@ redo:
 	}
 
 	if (!ld_moved) {
-		int active_balance;
+		int active_balance = 0;
 
 		schedstat_inc(sd, lb_failed[CPU_NEWLY_IDLE]);
 		if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
--
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