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

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Message-ID: <20130923170400.GA1390@linux.vnet.ibm.com>
Date:	Mon, 23 Sep 2013 10:04:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 06:01:30PM +0200, Peter Zijlstra wrote:
> 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().

Heh!  You want the old-style SRCU.  ;-)

> > 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.

At some point I suspect that we will want some form of fairness, but in
the meantime, good point.

							Thanx, Paul

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