Date: Tue, 3 Nov 2009 17:07:34 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Ian Campbell <Ian.Campbell@citrix.com>, Tejun Heo <tj@kernel.org>,
       "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Rusty Russell <rusty@rustcorp.com.au>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Correct nr_processes() when CPUs have been unplugged
Message-ID: <20091103160734.GA21362@elte.hu>
References: <1257243074.23110.779.camel@zakaz.uk.xensource.com>
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* Ian Campbell <Ian.Campbell@citrix.com> wrote:

> I'm not 100% convinced that the per_cpu regions remain valid for 
> offline CPUs, although my testing suggests that they do. If not then I 
> think the correct solution would be to aggregate the process_count for 
> a given CPU into a global base value in cpu_down().

Sidenote: percpu areas currently are kept allocated on x86.

That might change in the future though, especially with virtual systems 
where the possible range of CPUs can be very high - without us 
necessarily wanting to pay the percpu area allocation price for it. I.e. 
dynamic deallocation of percpu areas is something that could happen in 
the future.

> This bug appears to pre-date the transition to git and it looks like 
> it may even have been present in linux-2.6.0-test7-bk3 since it looks 
> like the code Rusty patched in http://lwn.net/Articles/64773/ was 
> already wrong.

Nice one. I'm wondering why it was not discovered for such a long time. 
That count can go out of sync easily, and we frequently offline cpus 
during suspend/resume, and /proc lookup failures will be noticed in 
general. How come nobody ran into this? And i'm wondering how you have 
run into this - running cpu hotplug stress-tests with Xen guests - or 
via pure code review?

	Ingo
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