linux-kernel - Re: [PATCH 11/15] sched: Pass unlimited __cpu_power information to upper domain level groups

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Date:	Mon, 24 Aug 2009 23:49:21 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andreas Herrmann <andreas.herrmann3@....com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Gautham Shenoy <ego@...ibm.com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH 11/15] sched: Pass unlimited __cpu_power information to
	upper domain level groups

* Peter Zijlstra <peterz@...radead.org> [2009-08-24 19:26:50]:

> On Mon, 2009-08-24 at 22:14 +0530, Balbir Singh wrote:
> > * Peter Zijlstra <peterz@...radead.org> [2009-08-24 17:21:37]:
> > 
> > > On Thu, 2009-08-20 at 15:41 +0200, Andreas Herrmann wrote:
> > > > For performance reasons __cpu_power in a sched_group might be limited
> > > > such that the group can handle only one task. To correctly calculate
> > > > the capacity in upper domain level groups the unlimited power
> > > > information is required. This patch stores unlimited __cpu_power
> > > > information in sched_groups.orig_power and uses this when calculating
> > > > __cpu_power in upper domain level groups.
> > > 
> > > OK, so this tries to fix the cpu_power wreckage?
> > > 
> > > ok, so let me try this with an example:
> > > 
> > > 
> > > Suppose we have a dual-core with shared cache and SMT
> > > 
> > >   0-3     MC
> > > 0-1 2-3   SMT
> > > 
> > > Then both levels fancy setting SHARED_RESOURCES and both levels end up
> > > normalizing the cpu_power to 1, so when we unplug cpu 2, load-balancing
> > > gets all screwy because the whole system doesn't get normalized
> > > properly.
> > > 
> > > What you propose here is every time we muck with cpu_power we keep the
> > > real stuff in orig_power and use that to compute the level above.
> > > 
> > > Except you don't use it in the load-balancer proper, so normalization is
> > > still hosed.
> > > 
> > > Its a creative solution, but I'd rather see cpu_power returned to a
> > > straight sum of actual power to normalize the inter-cpu runqueue weights
> > > and do the placement decision using something else.
> > 
> > The real solution is to find a way to solve asymmetric load balancing,
> > I suppose. The asymmetry might be due to cores being hot-plugged for
> > example
> 
> No, the solution is to not use cpu_power for placement and use it for
> normalization of the weight only. That would make the asym work by
> definition.
> 
> The real fun comes when we then introduce dynamic cpu_power based on
> feedback from things like aperf/mperf ratios for SMT and feedback from
> the RT scheduler.
>

That reminds me, accounting is currently broken and should be based on
APER/MPERF (Power gets it right - based on SPURR).
 
> The trouble is that cpu_power is now abused for placement decisions too,
> and that needs to be taken out.

OK.. so you propose extending the static cpu_power to dynamic
cpu_power but based on current topology?

-- 
	Balbir
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