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

[Mel Gorman]

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