[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <1220095613.8426.22.camel@twins>
Date: Sat, 30 Aug 2008 13:26:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Max Krasnyansky <maxk@...lcomm.com>
Cc: "svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>, 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>,
Gregory Haskins <ghaskins@...ell.com>
Subject: Re: sched_mc_power_savings broken with CGROUPS+CPUSETS
On Fri, 2008-08-29 at 13:29 -0700, Max Krasnyansky wrote:
> Peter Zijlstra wrote:
> > 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 do not fully understand how balancing is affected by the MC stuff but I can
> explain how the mc power saving settings are applied to the domains and the
> overall mechanism for that.
> Here a quote from one of my emails to Paul
>
> > Max wrote:
> > ...
> > Those things (mc_power and topology updates) have to update domain flags based
> > on the mc/smt power and current topology settings.
> > This is done in the
> > __rebuild_sched_domains()
> > ...
> > SD_INIT(sd, ALLNODES);
> > ...
> > SD_INIT(sd, MC);
> > ...
> >
> > SD_INIT(sd,X) uses one of SD initializers defined in the include/linux/topology.h
> > For example SD_CPU_INIT() includes BALANCE_FOR_PKG_POWER which expands to
> >
> > #define BALANCE_FOR_PKG_POWER \
> > ((sched_mc_power_savings || sched_smt_power_savings) ? \
> > SD_POWERSAVINGS_BALANCE : 0)
> >
> > Yes it's kind convoluted :). Anyway, the point is that we need to rebuild the
> > domains when those settings change. We could probably write a simpler version
> > that just iterates existing domains and updates the flags. Maybe some other dat :)
I don't think iterating the domains and setting the flag is sufficient.
Look at this crap (found in arch/x86/kernel/smpboot.c):
cpumask_t cpu_coregroup_map(int cpu)
{
struct cpuinfo_x86 *c = &cpu_data(cpu);
/*
* For perf, we return last level cache shared map.
* And for power savings, we return cpu_core_map
*/
if (sched_mc_power_savings || sched_smt_power_savings)
return per_cpu(cpu_core_map, cpu);
else
return c->llc_shared_map;
}
which means we'll actually end up building different domain/group
configurations depending on power savings settings.
> As I explained in the previous reply I missed the fact the logic that avoids
> redundant rebuilds in partition_sched_domains() will prevent
> arch_reinit_sched_domains() from doing the actual rebuild and hence will not
> apply the SD_POWERSAVINGS_BALANCE until something changes in cpuset setup.
>
> btw I can certainly attest to the fact that powersaving code is very hard to
> read and comprehend :)
Yeah - I was primarity hinting at the sched_group and find_*_group()
fudge, esp find_busiest_group() is an utter nightmare.
I'm still struggeling to understand _why_ we need those group things to
begin with, why aren't the child domains good enough?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists