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

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Date:	Sat, 27 Feb 2010 21:21:27 +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
Subject: Re: [PATCHv4 2/2] powerpc: implement arch_scale_smt_power for Power7

In message <11927.1267010024@...ling.org> you wrote:
> > > 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.  

I'm finding this SMT2 result to be unreliable. Sometimes it doesn't work
for the simple 1 process case.  It seems to change boot to boot.
Sometimes it works as expected with t0 busy and t1 idle, but other times
it's the other way around.

When it doesn't work, check_asym_packing() is still marking processes to
be pulled down but only gets run about 1 in every 4 calls to
load_balance().

For 2 of the other calls to load_balance, idle is CPU_NEWLY_IDLE and
hence check_asym_packing() doesn't get called.  This results in
sd->nr_balance_failed being reset.  When load_balance is next called and
check_asym_packing() hits, need_active_balance() returns 0 as
sd->nr_balance_failed is too small.  This means the migration thread on
t1 is not woken and the process remains there.  

So why does thread0 change from NEWLY_IDLE to IDLE and visa versa, when
there is nothing running on it?  Is this expected? 

This is with next-20100225 which pulled in Ingos tip at
407b4844f2af416cd8c9edd7c44d1545c93a4938 (from 24/2/2010)

Mikey
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