[patch] Re: hackbench regression with kernel 2.6.32-rc1

[Mike Galbraith <efault@gmx.de>]

On Thu, 2009-10-29 at 14:26 +0800, Zhang, Yanmin wrote: 
> On Thu, 2009-10-29 at 06:46 +0100, Mike Galbraith wrote:

> > SD_PREFER_LOCAL is still on in rc1 though (double checks;), so you'll go
> > through the power saving code until you reach a domain containing both
> > waker's cpu and wakee's previous cpu even if that code already found
> > that a higher domain wasn't overloaded.  Looks to me like that block
> > wants a want_sd && qualifier. 
> > 
> > Even it you turn SD_PREFER_LOCAL off, you can still hit the overhead if
> > SD_POWERSAVINGS_BALANCE is set, so I'd make sure both are off and see if
> > that's the source (likely, since the rest is already off).
> Yes. SD_POWERSAVINGS_BALANCE is disabled by default. I applied Peter's patch which
> turning SD_PREFER_LOCAL off for MC and cpu domain and it doesn't help.
> perf counter shows select_task_rq_fair still consumes about 5% cpu time. Eventually,
> I found for_each_cpu in for_each_domain consumes the 5% cpu time, because Peter's
> patch doesn't turn off SD_PREFER_LOCAL for node domain.
> I turned it off for node domain against the latest tips tree and tbench regression
> disappears on a Nehalem machine and becomes about 2% on another one.
> 
> Can we turn it off for node domain by default?

If it's hurting fast path overhead to the tune of an order of magnitude,
I guess there's no choice but to either fix it or turn it off.  Since
SD_BALANCE_WAKE is off globally, I don't see any point in keeping
SD_PREFER_LOCAL at any level.

(That said, what we need is for this to not be so expensive that we
can't afford it in the fast path).

sched: Disable SD_PREFER_LOCAL at node level.

Yanmin Zhang reported that SD_PREFER_LOCAL induces an order of magnitude
increase in select_task_rq_fair() overhead while running heavy wakeup
benchmarks (tbench and vmark).  Since SD_BALANCE_WAKE is off at node level,
turn SD_PREFER_LOCAL off as well pending further investigation. 

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Zhang, Yanmin <yanmin_zhang@linux.intel.com>
LKML-Reference: <new-submission>

diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index d823c24..40e37b1 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -143,7 +143,7 @@ extern unsigned long node_remap_size[];
 				| 1*SD_BALANCE_FORK			\
 				| 0*SD_BALANCE_WAKE			\
 				| 1*SD_WAKE_AFFINE			\
-				| 1*SD_PREFER_LOCAL			\
+				| 0*SD_PREFER_LOCAL			\
 				| 0*SD_SHARE_CPUPOWER			\
 				| 0*SD_POWERSAVINGS_BALANCE		\
 				| 0*SD_SHARE_PKG_RESOURCES		\