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

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Message-Id: <1251134810.7538.320.camel@twins>
Date:	Mon, 24 Aug 2009 19:26:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	balbir@...ux.vnet.ibm.com
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

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.

The trouble is that cpu_power is now abused for placement decisions too,
and that needs to be taken out.
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