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Date: Wed, 9 Oct 2013 22:47:55 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
        Ingo Molnar <mingo@kernel.org>, 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 1/6] hotplug: Optimize {get,put}_online_cpus()
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On Tue, 08 Oct 2013 12:25:06 +0200 Peter Zijlstra <peterz@infradead.org> wrote:

> The current implementation of get_online_cpus() is global of nature
> and thus not suited for any kind of common usage.
> 
> Re-implement the current recursive r/w cpu hotplug lock such that the
> read side locks are as light as possible.
> 
> The current cpu hotplug lock is entirely reader biased; but since
> readers are expensive there aren't a lot of them about and writer
> starvation isn't a particular problem.
> 
> However by making the reader side more usable there is a fair chance
> it will get used more and thus the starvation issue becomes a real
> possibility.
> 
> Therefore this new implementation is fair, alternating readers and
> writers; this however requires per-task state to allow the reader
> recursion.

Obvious question: can't we adapt lglocks for this?  It would need the
counter in task_struct to permit reader nesting, but what else is
needed?