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    SubjectRe: sched_mc_power_savings broken with CGROUPS+CPUSETS
    * Peter Zijlstra <peterz@infradead.org> [2008-08-29 15:23:57]:

    > 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.

    I will try to add my notes around it. I have been playing around with
    sched_mc to make it work for different workloads.

    --Vaidy



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