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

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Date:	Mon, 30 Sep 2013 20:56:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.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>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Fri, Sep 27, 2013 at 10:41:16PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 27, 2013 at 08:15:32PM +0200, Oleg Nesterov wrote:
> > On 09/26, Peter Zijlstra wrote:

[ . . . ]

> > > +static bool cpuhp_readers_active_check(void)
> > >  {
> > > +	unsigned int seq = per_cpu_sum(cpuhp_seq);
> > > +
> > > +	smp_mb(); /* B matches A */
> > > +
> > > +	/*
> > > +	 * In other words, if we see __get_online_cpus() cpuhp_seq increment,
> > > +	 * we are guaranteed to also see its __cpuhp_refcount increment.
> > > +	 */
> > >  
> > > +	if (per_cpu_sum(__cpuhp_refcount) != 0)
> > > +		return false;
> > >  
> > > +	smp_mb(); /* D matches C */
> > 
> > It seems that both barries could be smp_rmb() ? I am not sure the comments
> > from srcu_readers_active_idx_check() can explain mb(), note that
> > __srcu_read_lock() always succeeds unlike get_cpus_online().
> 
> I see what you mean; cpuhp_readers_active_check() is all purely reads;
> there are no writes to order.
> 
> Paul; is there any argument for the MB here as opposed to RMB; and if
> not should we change both these and SRCU?

Given that these memory barriers execute only on the semi-slow path,
why add the complexity of moving from smp_mb() to either smp_rmb()
or smp_wmb()?  Straight smp_mb() is easier to reason about and more
robust against future changes.

							Thanx, Paul

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