linux-kernel - Re: [PATCH 04/25] mm, vmscan: Begin reclaiming pages on a per-node basis

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Date:	Mon, 15 Jun 2015 09:22:59 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Hillf Danton <hillf.zj@...baba-inc.com>
Cc:	linux-mm@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 04/25] mm, vmscan: Begin reclaiming pages on a per-node
 basis

On Thu, Jun 11, 2015 at 03:58:14PM +0800, Hillf Danton wrote:
> > @@ -1319,6 +1322,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> >  	struct list_head *src = &lruvec->lists[lru];
> >  	unsigned long nr_taken = 0;
> >  	unsigned long scan;
> > +	LIST_HEAD(pages_skipped);
> > 
> >  	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
> >  		struct page *page;
> > @@ -1329,6 +1333,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> > 
> >  		VM_BUG_ON_PAGE(!PageLRU(page), page);
> > 
> > +		if (page_zone_id(page) > sc->reclaim_idx)
> > +			list_move(&page->lru, &pages_skipped);
> > +
> >  		switch (__isolate_lru_page(page, mode)) {
> >  		case 0:
> >  			nr_pages = hpage_nr_pages(page);
> > @@ -1347,6 +1354,15 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> >  		}
> >  	}
> > 
> > +	/*
> > +	 * Splice any skipped pages to the start of the LRU list. Note that
> > +	 * this disrupts the LRU order when reclaiming for lower zones but
> > +	 * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
> > +	 * scanning would soon rescan the same pages to skip and put the
> > +	 * system at risk of premature OOM.
> > +	 */
> > +	if (!list_empty(&pages_skipped))
> > +		list_splice(&pages_skipped, src);
> >  	*nr_scanned = scan;
> >  	trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
> >  				    nr_taken, mode, is_file_lru(lru));
> 
> Can we avoid splicing pages by skipping pages with scan not incremented?
> 

The reclaimers would still have to do the work of examining those pages
and ignoring them even if the counters are not updated. It'll look like
high CPU usage for no obvious reason.

-- 
Mel Gorman
SUSE Labs
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