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Date: Wed, 2 Oct 2013 15:31:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
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To: Oleg Nesterov <oleg@redhat.com>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Steven Rostedt <rostedt@goodmis.org>, Viresh Kumar <viresh.kumar@linaro.org>

On Wed, Oct 02, 2013 at 02:13:56PM +0200, Oleg Nesterov wrote:
> In short: unless a gp elapses between _exit() and _enter(), the next
> _enter() does nothing and avoids synchronize_sched().

That does however make the entire scheme entirely writer biased;
increasing the need for the waitcount thing I have. Otherwise we'll
starve pending readers.

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