linux-kernel - Re: [PATCH 11/22] mm, compaction: Use free lists to quickly locate a migration target

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Message-ID: <20190201145139.GI9565@techsingularity.net>
Date:   Fri, 1 Feb 2019 14:51:39 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 11/22] mm, compaction: Use free lists to quickly locate a
 migration target

On Thu, Jan 31, 2019 at 03:52:10PM +0100, Vlastimil Babka wrote:
> > -/* Reorder the free list to reduce repeated future searches */
> > +/*
> > + * Used when scanning for a suitable migration target which scans freelists
> > + * in reverse. Reorders the list such as the unscanned pages are scanned
> > + * first on the next iteration of the free scanner
> > + */
> > +static void
> > +move_freelist_head(struct list_head *freelist, struct page *freepage)
> > +{
> > +	LIST_HEAD(sublist);
> > +
> > +	if (!list_is_last(freelist, &freepage->lru)) {
> 
> Shouldn't there be list_is_first() for symmetry?
> 

I don't think it would help. We're reverse traversing the list when this is
called. If it's the last entry, it's moving just one page before breaking
off the search and a shuffle has minimal impact. If it's the first page
then list_cut_before moves the entire list to sublist before splicing it
back so it's a pointless operation.

-- 
Mel Gorman
SUSE Labs