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

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Message-ID: <20130923160130.GC9326@twins.programming.kicks-ass.net>
Date:	Mon, 23 Sep 2013 18:01:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Mon, Sep 23, 2013 at 08:50:59AM -0700, Paul E. McKenney wrote:
> Not a problem, just stuff the idx into some per-task thing.  Either
> task_struct or taskinfo will work fine.

Still not seeing the point of using srcu though..

srcu_read_lock() vs synchronize_srcu() is the same but far more
expensive than preempt_disable() vs synchronize_sched().

> Or to put it another way, if the underlying slow-path mutex is
> reader-preference, then the whole thing will be reader-preference.

Right, so 1) we have no such mutex so we're going to have to open-code
that anyway, and 2) like I just explained in the other email, I want the
pending writer case to be _fast_ as well.
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