Re: sched_mc_power_savings broken with CGROUPS+CPUSETS

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2008-08-29 at 18:45 +0530, Vaidyanathan Srinivasan wrote:
> 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.

I'm still greatly mistified by all that power savings code.

Its hard to read and utterly hard to comprehend - I've been about to rip
the whole stuff out on several occasions. But so far tried to carefully
thread around it maintaining its operation even though not fully
understood.

Someone with clue - preferably the authors of the code in question -
should enlighten us with a patch that adds some comments as to the
intent of said lines of code.