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From: Vaidyanathan Srinivasan on 8 Feb 2010 05:10
Hi Peter,

sched_mc_powersavings is broken in pre-Nehalem x86 platforms due to
contradictory SD flags at MC level and CPU level. SD_PREFER_SIBLING being set
at MC level is expected to do the following:

a) Disable consolidating tasks to single group in the parent sched domain
(generally single cpu package)

b) Spread tasks equally across groups at the parent sched domain.

While SD_POWERSAVINGS_BALANCE set at a sched domain will enable logic to
consolidate tasks within minimum number of groups at that sched domain.

Basically SD_POWERSAVINGS_BALANCE at one sched domain and its child domain
having SD_PREFER_SIBLING is contradicting and disabling the
SD_POWERSAVINGS_BALANCE logic in

if (local_group && (sds->this_nr_running >= sgs->group_capacity ||
!sds->this_nr_running))
sds->power_savings_balance = 0;

Since sgs.group_capacity is set to '1' by SD_PREFER_SIBLING in child
sched domain.

The attached patch will fix the expected behavior for sched_mc_powersavings > 0
while objective (b) is still an open issue.

The following condition in find_busiest_group()
sds.max_load <= sds.busiest_load_per_task

treats unequally loaded groups as balanced as longs they are below
capacity

Test Results:

The following patch was tested on dual socket quad core non-threaded Xeon:

Running 4 while(1) loops in shell:

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

Without Patch:
Running 1 task in one quad core package and 3 in another.
This is effectively the baseline behavior with sched_mc=0

With patch:
All 4 tasks running in one quad core package.
Expected behavior for sched_mc_powersavings>0

--Vaidy

Fix for sched_mc_powersavigs for pre-Nehalem platforms.
Child sched domain should clear SD_PREFER_SIBLING if parent will have
SD_POWERSAVINGS_BALANCE because they are contradicting.

Sets the flags correctly based on sched_mc_power_savings.

Signed-off-by: Vaidyanathan Srinivasan <svaidy(a)linux.vnet.ibm.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6550415..ef6b7cd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -866,7 +866,10 @@ static inline int sd_balance_for_mc_power(void)
if (sched_smt_power_savings)
return SD_POWERSAVINGS_BALANCE;

- return SD_PREFER_SIBLING;
+ if (!sched_mc_power_savings)
+ return SD_PREFER_SIBLING;
+
+ return 0;
}

static inline int sd_balance_for_package_power(void)

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From: Peter Zijlstra on 8 Feb 2010 06:40
On Mon, 2010-02-08 at 15:35 +0530, Vaidyanathan Srinivasan wrote:

> Fix for sched_mc_powersavigs for pre-Nehalem platforms.
> Child sched domain should clear SD_PREFER_SIBLING if parent will have
> SD_POWERSAVINGS_BALANCE because they are contradicting.
>
> Sets the flags correctly based on sched_mc_power_savings.
>
> Signed-off-by: Vaidyanathan Srinivasan <svaidy(a)linux.vnet.ibm.com>
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 6550415..ef6b7cd 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -866,7 +866,10 @@ static inline int sd_balance_for_mc_power(void)
> if (sched_smt_power_savings)
> return SD_POWERSAVINGS_BALANCE;
>
> - return SD_PREFER_SIBLING;
> + if (!sched_mc_power_savings)
> + return SD_PREFER_SIBLING;
> +
> + return 0;
> }
>
> static inline int sd_balance_for_package_power(void)
>

Looks good, thanks!

What's the status of getting rid of sched_{mc,smt}_power_savings?

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From: Vaidyanathan Srinivasan on 8 Feb 2010 07:50
* Peter Zijlstra <peterz(a)infradead.org> [2010-02-08 12:35:48]:

> On Mon, 2010-02-08 at 15:35 +0530, Vaidyanathan Srinivasan wrote:
>
> > Fix for sched_mc_powersavigs for pre-Nehalem platforms.
> > Child sched domain should clear SD_PREFER_SIBLING if parent will have
> > SD_POWERSAVINGS_BALANCE because they are contradicting.
> >
> > Sets the flags correctly based on sched_mc_power_savings.
> >
> > Signed-off-by: Vaidyanathan Srinivasan <svaidy(a)linux.vnet.ibm.com>
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 6550415..ef6b7cd 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -866,7 +866,10 @@ static inline int sd_balance_for_mc_power(void)
> > if (sched_smt_power_savings)
> > return SD_POWERSAVINGS_BALANCE;
> >
> > - return SD_PREFER_SIBLING;
> > + if (!sched_mc_power_savings)
> > + return SD_PREFER_SIBLING;
> > +
> > + return 0;
> > }
> >
> > static inline int sd_balance_for_package_power(void)
> >
>
> Looks good, thanks!
>
> What's the status of getting rid of sched_{mc,smt}_power_savings?

Hi Peter,

With the current rearrangement of the code, the unified
sched_power_savings seems more doable.

However, I have few more fixes for sched_smt_powersavings on Nehalem
before I would revisit the unified tunable.

--Vaidy

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Prev: i915 / PM: Fix crash while aborting hibernation (Re: [linux-pm] [regression] "drm/i915: implement new pm ops" disables irq on aborted s2disk)
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