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Date: Thu, 10 Oct 2013 16:57:38 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Oleg Nesterov <oleg@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
Message-ID: <20131010145738.GA5167@gmail.com>
References: <20131008102505.404025673@infradead.org>
 <20131009225006.7101379c.akpm@linux-foundation.org>
 <20131010121908.GB28601@twins.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> But the thing is; our sense of NR_CPUS has shifted, where it used to be 
> ok to do something like:
> 
>   for_each_cpu()
> 
> With preemption disabled; it gets to be less and less sane to do so, 
> simply because 'common' hardware has 256+ CPUs these days. If we cannot 
> rely on preempt disable to exclude hotplug, we must use 
> get_online_cpus(), but get_online_cpus() is global state and thus cannot 
> be used at any sort of frequency.

So ... why not make it _really_ cheap, i.e. the read lock costing nothing, 
and tie CPU hotplug to freezing all tasks in the system?

Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a 
system, I don't understand how we tolerate _any_ overhead from this utter 
slowpath.

Thanks,

	Ingo