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SubjectRe: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations
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On Sat, 2020-03-07 at 14:38 -0800, Andrew Morton wrote:
> On Fri, 6 Mar 2020 15:01:02 -0500 Rik van Riel <riel@surriel.com>
> wrote:

> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2711,6 +2711,18 @@ __rmqueue(struct zone *zone, unsigned int
> > order, int migratetype,
> > {
> > struct page *page;
> >
> > + /*
> > + * Balance movable allocations between regular and CMA areas by
> > + * allocating from CMA when over half of the zone's free memory
> > + * is in the CMA area.
> > + */
> > + if (migratetype == MIGRATE_MOVABLE &&
> > + zone_page_state(zone, NR_FREE_CMA_PAGES) >
> > + zone_page_state(zone, NR_FREE_PAGES) / 2) {
> > + page = __rmqueue_cma_fallback(zone, order);
> > + if (page)
> > + return page;
> > + }
> > retry:
> > page = __rmqueue_smallest(zone, order, migratetype);
> > if (unlikely(!page)) {
>
> __rmqueue() is a hot path (as much as any per-page operation can be a
> hot path). What is the impact here?

That is a good question. For MIGRATE_MOVABLE allocations,
most allocations seem to be order 0, which go through the
per cpu pages array, and rmqueue_pcplist, or be order 9.

For order 9 allocations, other things seem likely to dominate
the allocation anyway, while for order 0 allocations the
pcp list should take away the sting?

What I do not know is how much impact this change would
have on other allocations, like order 3 or order 4 network
buffer allocations from irq context...

Are there cases in particular that we should be testing?

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