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

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Message-ID: <20081119203340.GC2383@sgi.com>
Date:	Wed, 19 Nov 2008 14:33:40 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Max Krasnyansky <maxk@...lcomm.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 Wed, Nov 19, 2008 at 03:25:15PM -0500, Gregory Haskins wrote:
> It sounds like the problem with my code is that "null sched domain"
> translates into "default root-domain" which is understandably unexpected
> by Dimitri (and myself).  Really I intended root-domains to become
> associated with each exclusive/disjoint cpuset that is created.  In a
> way, non-balanced/isolated cpus could be modeled as an exclusive cpuset
> with one member, but that is somewhat beyond the scope of the

Actually, at one time, that is how things were setup.  Setting the
cpu_exclusive bit on a single cpu cpuset would isolate that cpu from
load balancing.

> root-domain code as it stands today.  My primary concern was that
> Dimitri reports that even creating a disjoint cpuset per cpu does not
> yield an isolated root-domain per cpu.  Rather they all end up in the
> default root-domain, and this is not what I intended at all.
> 
> However, as a secondary goal it would be nice to somehow directly
> support the "no-load-balance" option without requiring explicit
> exclusive per-cpu cpusets to do it.  The proper mechanism (IMHO) to
> scope the scheduler to a subset of cpus (including only "self") is
> root-domains so I would prefer to see the solution based on that. 
> However, today there is a rather tight coupling of root-domains and
> cpusets, so this coupling would likely have to be relaxed a little bit
> to get there.
> 
> There are certainly other ways to solve the problem as well.  But seeing
> as how I intended root-domains to represent the effective partition
> scope of the scheduler, this seems like a natural fit in my mind until
> its proven to me otherwise.
>

Agreed. 
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