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Date: Thu, 10 Oct 2013 18:53:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Thu, Oct 10, 2013 at 09:43:55AM -0700, Andrew Morton wrote:
> > But we would like to remove stomp machine from
> > CPU hotplug.
> 
> We do?  That's news.  It wasn't mentioned in the changelog and should
> have been.  Why?  

It would be an unrelated change to this and unrelated to the reasons as
to why I want a faster get_online_cpus().

> > Why prevent all CPUs from running when we want to remove
> > one?
> 
> So get_online_cpus() goes away.  Nothing is more scalable than nothing!

Very much agreed; now stop_machine() wouldn't actually work for hotplug
because it will instantly preempt everybody, including someone who might
be in the middle of using per-cpu state of the cpu we're about to
remove.

The freeze suggestion from Ingo would actually work because we freeze
tasks at known good points (userspace and kthread_freeze() points) where
we know they're not fiddling with per-cpu state.