linux-kernel - Re: [PATCHv4 2/2] powerpc: implement arch_scale_smt

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Message-ID: <11927.1267010024@neuling.org>
Date:	Wed, 24 Feb 2010 22:13:44 +1100
From:	Michael Neuling <mikey@...ling.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Joel Schopp <jschopp@...tin.ibm.com>, Ingo Molnar <mingo@...e.hu>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	ego@...ibm.com, Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCHv4 2/2] powerpc: implement arch_scale_smt_power for Power7

> > If there's less the group will normally be balanced and we fall out and
> > end up in check_asym_packing().
> > 
> > So what I tried doing with that loop is detect if there's a hole in the
> > packing before busiest. Now that I think about it, what we need to check
> > is if this_cpu (the removed cpu argument) is idle and less than busiest.
> > 
> > So something like:
> > 
> > static int check_asym_pacing(struct sched_domain *sd,
> >                              struct sd_lb_stats *sds,
> >                              int this_cpu, unsigned long *imbalance)
> > {
> > 	int busiest_cpu;
> > 
> > 	if (!(sd->flags & SD_ASYM_PACKING))
> > 		return 0;
> > 
> > 	if (!sds->busiest)
> > 		return 0;
> > 
> > 	busiest_cpu = group_first_cpu(sds->busiest);
> > 	if (cpu_rq(this_cpu)->nr_running || this_cpu > busiest_cpu)
> > 		return 0;
> > 
> > 	*imbalance = (sds->max_load * sds->busiest->cpu_power) /
> > 			SCHED_LOAD_SCALE;
> > 	return 1;
> > }
> > 
> > Does that make sense?
> 
> I think so.
> 
> I'm seeing check_asym_packing do the right thing with the simple SMT2
> with 1 process case.  It marks cpu0 as imbalanced when cpu0 is idle and
> cpu1 is busy.
> 
> Unfortunately the process doesn't seem to be get migrated down though.
> Do we need to give *imbalance a higher value? 

So with ego help, I traced this down a bit more.  

In my simple test case (SMT2, t0 idle, t1 active) if f_b_g() hits our
new case in check_asym_packing(), load_balance then runs f_b_q().
f_b_q() has this:

  		if (capacity && rq->nr_running == 1 && wl > imbalance)
			continue;

when check_asym_packing() hits, wl = 1783 and imbalance = 1024, so we
continue and busiest remains NULL. 

load_balance then does "goto out_balanced" and it doesn't attempt to
move the task.

Based on this and on egos suggestion I pulled in Suresh Siddha patch
from: http://lkml.org/lkml/2010/2/12/352.  This fixes the problem.  The
process is moved down to t0.  

I've only tested SMT2 so far.  

Mikey
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