Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used during runtime

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used during runtime
Date	2015-07-17 15:30 +0200
Message-ID	<pNdSa-6GQ-13@gated-at.bofh.it> (permalink)
References	<pNcW6-5lc-7@gated-at.bofh.it> <pNcW6-5lc-13@gated-at.bofh.it> <pNdIv-6vu-29@gated-at.bofh.it> <pNdIu-6vu-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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?
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Thread

[PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used during runtime Mel Gorman <mgorman@suse.de> - 2015-07-17 14:30 +0200
  Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used  during runtime Mel Gorman <mgorman@suse.de> - 2015-07-17 15:20 +0200
    Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used  during runtime Peter Zijlstra <peterz@infradead.org> - 2015-07-17 15:30 +0200
      Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used  during runtime Mel Gorman <mgorman@suse.de> - 2015-07-17 15:40 +0200
        Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used  during runtime Peter Zijlstra <peterz@infradead.org> - 2015-07-17 16:00 +0200
  Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used  during runtime Peter Zijlstra <peterz@infradead.org> - 2015-07-17 15:20 +0200

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