linux-kernel - Re: [patch v2] mm, tmp: only collapse hugepages to nodes with affinity for zone_reclaim

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Date:	Wed, 16 Jul 2014 17:49:37 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Bob Liu <bob.liu@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v2] mm, tmp: only collapse hugepages to nodes with affinity
 for zone_reclaim_mode

On Wed, 16 Jul 2014, Vlastimil Babka wrote:

> >> @@ -2545,6 +2571,11 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >>  		 * hit record.
> >>  		 */
> >>  		node = page_to_nid(page);
> >> +		if (node != last_node) {
> >> +			if (khugepaged_scan_abort(node))
> >> +				goto out_unmap;
> > 
> > Nitpick: How about not break the loop but only reset the related
> > khugepaged_node_load[] to zero. E.g. modify khugepaged_scan_abort() like
> > this:
> > if (node_distance(nid, i) > RECLAIM_DISTANCE)
> >    khugepaged_node_load[i] = 0;
> > 
> > By this way, we may have a chance to find a more suitable node.
> 
> Hm theoretically there might be a suitable node, but this approach wouldn't
> work. By resetting it to zero you forget that there ever was node 'i'. If there
> is no more base page from node 'i', the load remains zero and the next call with
> 'nid' will think that 'nid' is OK.
> 

Right, the suggestion is wrong because we do not want to ever collapse to 
a node when the distance from the source page is > RECLAIM_DISTANCE, 
that's the entire point of the patch.
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