From: Rusty Russell <rusty@rustcorp.com.au>
To: Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH] Correct nr_processes() when CPUs have been unplugged
Date: Thu, 5 Nov 2009 11:13:33 +1030
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Cc: Ingo Molnar <mingo@elte.hu>, Ian Campbell <Ian.Campbell@citrix.com>,
       Tejun Heo <tj@kernel.org>, "Paul E. McKenney" <paulmck@us.ibm.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       "linux-kernel" <linux-kernel@vger.kernel.org>
References: <1257243074.23110.779.camel@zakaz.uk.xensource.com> <20091103160734.GA21362@elte.hu> <alpine.DEB.1.10.0911031324380.32136@V090114053VZO-1>
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On Wed, 4 Nov 2009 05:04:32 am Christoph Lameter wrote:
> On Tue, 3 Nov 2009, Ingo Molnar wrote:
> 
> > Sidenote: percpu areas currently are kept allocated on x86.
> 
> They must be kept allocated for all possible cpus. Arch code cannot decide
> to not allocate per cpu areas.
> 
> Search for "for_each_possible_cpu" in the source tree if you want more
> detail.

Yeah, handling onlining/offlining of cpus is a hassle for most code.

But I can see us wanting abstractions for counters which handle being per-cpu
or per-node and doing the folding etc. automagically.  It's best that this
be done by looking at all the existing users to see if there's a nice API
which would cover 90%.

Cheers,
Rusty.
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