linux-kernel - Re: RT sched: cpupri_vec lock contention with def_root

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Message-ID: <20081124151113.GB2292@sgi.com>
Date:	Mon, 24 Nov 2008 09:11:13 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Li Zefan <lizf@...fujitsu.com>,
	Gregory Haskins <ghaskins@...ell.com>
Cc:	Max Krasnyansky <maxk@...lcomm.com>, Derek Fults <dfults@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: RT sched: cpupri_vec lock contention with def_root_domain and no load balance

On Sat, Nov 22, 2008 at 04:18:29PM +0800, Li Zefan wrote:
> Max Krasnyansky wrote:
> > 
> > Dimitri Sivanich wrote:
> >>
> >> Which is the way sched_load_balance is supposed to work. You need to set
> >> sched_load_balance=0 for all cpusets containing any cpu you want to disable
> >> balancing on, otherwise some balancing will happen.
> > It won't be much of a balancing in this case because this just one cpu per
> > domain.
> > In other words no that's not how it supposed to work. There is code in
> > cpu_attach_domain() that is supposed to remove redundant levels
> > (sd_degenerate() stuff). There is an explicit check in there for numcpus == 1.
> > btw The reason you got a different result that I did is because you have a
> > NUMA box where is mine is UMA. I was able to reproduce the problem though by
> > enabling multi-core scheduler. In which case I also get one redundant domain
> > level CPU, with a single CPU in it.
> > So we definitely need to fix this. I'll try to poke around tomorrow and figure
> > out why redundant level is not dropped.
> > 
> 
> You were not using latest kernel, were you?
> 
> There was a bug in sd degenerate code, and it has already been fixed:
> 	http://lkml.org/lkml/2008/11/8/10

With the above patch added, we now see the results that Max is showing as far as individual root domains being created with a span of just their own cpu when sched_load_balance is turned off.
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