linux-kernel - Re: [PATCH 1/6] hotplug: Optimize {get,put}_online

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Message-Id: <20131009224755.09d32206.akpm@linux-foundation.org>
Date:	Wed, 9 Oct 2013 22:47:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] hotplug: Optimize {get,put}_online_cpus()

On Tue, 08 Oct 2013 12:25:06 +0200 Peter Zijlstra <peterz@...radead.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?

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