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Date: Fri, 17 Jul 2015 15:29:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Nicolai Stange <nicstange@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>, Alex Ng <alexng@microsoft.com>,
        Fengguang Wu <fengguang.wu@intel.com>, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used
 during runtime
Message-ID: <20150717132922.GN19282@twins.programming.kicks-ass.net>
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On Fri, Jul 17, 2015 at 02:17:29PM +0100, Mel Gorman wrote:
> On Fri, Jul 17, 2015 at 03:12:32PM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 17, 2015 at 01:22:04PM +0100, Mel Gorman wrote:
> > >  int __meminit early_pfn_to_nid(unsigned long pfn)
> > >  {
> > > +	static DEFINE_SPINLOCK(early_pfn_lock);
> > >  	int nid;
> > >  
> > > -	/* The system will behave unpredictably otherwise */
> > > -	BUG_ON(system_state != SYSTEM_BOOTING);
> > > +	/* Avoid locking overhead during boot but hotplug must lock */
> > > +	if (system_state != SYSTEM_BOOTING)
> > > +		spin_lock(&early_pfn_lock);
> > >  
> > >  	nid = __early_pfn_to_nid(pfn, &early_pfnnid_cache);
> > > -	if (nid >= 0)
> > > -		return nid;
> > > -	/* just returns 0 */
> > > -	return 0;
> > > +	if (nid < 0)
> > > +		nid = 0;
> > > +
> > > +	if (system_state != SYSTEM_BOOTING)
> > > +		spin_unlock(&early_pfn_lock);
> > > +
> > > +	return nid;
> > >  }
> > 
> > Why the conditional locking?
> 
> Unnecessary during boot when it's inherently serialised. The point of
> the deferred initialisation was to boot as quickly as possible.

Sure, but does it make a measurable difference?