linux-kernel - Re: [RFC/PATCH] cpuset: cpuset irq affinities

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Message-Id: <20080304005148.4e6f1fe8.pj@sgi.com>
Date:	Tue, 4 Mar 2008 00:51:48 -0600
From:	Paul Jackson <pj@....com>
To:	"Paul Menage" <menage@...gle.com>
Cc:	a.p.zijlstra@...llo.nl, maxk@...lcomm.com, mingo@...e.hu,
	tglx@...utronix.de, oleg@...sign.ru, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, rientjes@...gle.com
Subject: Re: [RFC/PATCH] cpuset: cpuset irq affinities

Paul M wrote:
> I'm one such user who's been forced to add the mem_hardwall flag to
> get around the fact that exclusive and hardwall are controlled by the
> same flag. I keep meaning to send it in as a patch but haven't yet got
> round to it.

I made essentially the same mistake twice in the evolution of cpusets:
 1) overloading the cpu_exclusive flag to define sched domains, and
 2) overloading the mem_exclusive flag to define memory hardwalls.

I eventually reversed (1), with a deliberately incompatible change
(and you know how I resist those ;), creating a new 'sched_load_balance'
flag that controls the sched_domain partitioning, and removing any
affect that the cpu_exclusive flag has on this.

Perhaps the unfortunate interaction of mem_exclusive and hardwall is
destined to go the same path.  Thought the audience that is currently
using mem_exclusive for the purpose of hardwall enforcement of kernel
allocations might be broader than the specialized real-time audience
that was using cpu_exclusive for dynamic sched domain isolation, and so
we might not choose to just break compatibility in one shot, but rather
phase in your new flag, before, perhaps, in a later release, phasing
out the old hardwall overloading of the mem_exclusive flag.

(My primeval mistake was including the cpu_exclusive and mem_exclusive
flags in the original cpuset design; those two flags have given me
nothing but temptation to commit further design errors ;).

> Also, if you're using fake numa for memory isolation (which we're
> experimenting with) then the correlation between cpu placement and
> memory placement is much much weaker, or non-existent.

That might be a good answer to my asking where the beef was.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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