linux-kernel - Re: [PATCH] Correct nr_processes() when CPUs have been unplugged

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Message-Id: <200911051113.33889.rusty@rustcorp.com.au>
Date:	Thu, 5 Nov 2009 11:13:33 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Correct nr_processes() when CPUs have been unplugged

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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