linux-kernel - sched_mc_power_savings broken with CGROUPS+CPUSETS

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Date:	Fri, 29 Aug 2008 18:45:15 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: sched_mc_power_savings broken with CGROUPS+CPUSETS

Hi,

sched_mc_power_savings seems to be broken with CGROUPS+CPUSETS.
When CONFIG_CPUSETS=y the attached BUG_ON() is being hit.

I added a BUG_ON to check if SD_POWERSAVINGS_BALANCE is set at
SD_LV_CPU whenever sched_mc_power_savings is set.

This BUG is hit when config CONFIG_CPUSETS (depends on CONFIG_CGROUPS)
is just compiled in while this is never hit when they are compiled
out.  The fact that SD_POWERSAVINGS_BALANCE being cleared even when
sched_mc_power_savings = 1 completely breaks the
sched_mc_power_savings heuristics.

To recreate the problem, 
Have sched_mc power savings enabled CONFIG_SCHED_MC=y
Add this BUG_ON()

echo 1 > /sys/devices/system/cpu/sched_mc_power_savings

Try these these on a multi core x86 box.

sched_mc_power_savings seems to be broken from 2.6.26-rc1, but
I do not have a confirmation that the root cause is same in all
successive versions. sched_mc_power_savings works perfect in 
2.6.25.

Please help me root cause the issue.  Please point me to changes that
may potential cause this bug.

Thanks,
Vaidy

Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>

Index: linux-2.6.27-rc5/kernel/sched.c
===================================================================
--- linux-2.6.27-rc5.orig/kernel/sched.c	2008-08-29 17:48:56.000000000 +0000
+++ linux-2.6.27-rc5/kernel/sched.c	2008-08-29 18:16:09.000000000 +0000
@@ -3375,6 +3375,8 @@

 out_balanced:
 #if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
+	BUG_ON(sched_mc_power_savings && sd->level == SD_LV_CPU && 
+		!(sd->flags & SD_POWERSAVINGS_BALANCE)); 
 	if (idle == CPU_NOT_IDLE || !(sd->flags & SD_POWERSAVINGS_BALANCE))
 		goto ret;

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