linux-kernel - Re: [PATCH 2/2] S390 topology: don't use kthread() for arch_reinit_sched

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Date:	Mon, 30 Jun 2008 12:49:21 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Paul Jackson <pj@....com>
CC:	heiko.carstens@...ibm.com, oleg@...sign.ru,
	akpm@...ux-foundation.org, ego@...ibm.com, menage@...gle.com,
	peterz@...radead.org, vegard.nossum@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] S390 topology: don't use kthread() for arch_reinit_sched_domains()

Paul Jackson wrote:
> Max wrote:
>> we either have to ... or change arch_init_sched_domains() to no destroy 
>> current domains.
> 
> I might be misreading this, but I doubt that just not destroying
> current domains is an option.  Once any CPU goes on or off line, the
> only way back to the new correct sched domain configuration is via the
> rebuild_sched_domains() routine in kernel/cpuset.c.
> 

Despite all the typos and missing words you read it correctly :). Here 
is what I'm thinking.
When a CPU goes off line overall partitioning does not change we just 
need to update current domains and remove the CPU that is no longer 
available. When a CPU goes online it always ends up in the root cpuset, 
which means it can be added to the first load-balanced sched domain.

In other words I'm thinking of simulating what rebuild_sched_domains() 
would've done on hotplug events and calling partition_sched_domains() 
directly from sched cpu hotplug code.
That way we can avoid cpuset/cgroup locking in that path.
Now, I haven't really looked into details. Maybe it's not feasible. In 
which case Paul M.'s new locking scheme is the way to go.

Max
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