Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES approaches low water mark

Marek Szyprowski
2012-11-19T15:38:23Z

Hello,

On 11/14/2012 11:58 PM, Andrew Morton wrote:
> On Mon, 12 Nov 2012 09:59:42 +0100
> Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>
> > It has been observed that system tends to keep a lot of CMA free pages
> > even in very high memory pressure use cases. The CMA fallback for movable
> > pages is used very rarely, only when system is completely pruned from
> > MOVABLE pages, what usually means that the out-of-memory even will be
> > triggered very soon. To avoid such situation and make better use of CMA
> > pages, a heuristics is introduced which turns on CMA fallback for movable
> > pages when the real number of free pages (excluding CMA free pages)
> > approaches low water mark.
> >
> > Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
> > CC: Michal Nazarewicz <mina86@mina86.com>
> > ---
> > mm/page_alloc.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index fcb9719..90b51f3 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1076,6 +1076,15 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
> > {
> > struct page *page;
> >
> > +#ifdef CONFIG_CMA
> > + unsigned long nr_free = zone_page_state(zone, NR_FREE_PAGES);
> > + unsigned long nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
> > +
> > + if (migratetype == MIGRATE_MOVABLE && nr_cma_free &&
> > + nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
> > + migratetype = MIGRATE_CMA;
> > +#endif /* CONFIG_CMA */
> > +
> > retry_reserve:
> > page = __rmqueue_smallest(zone, order, migratetype);
>
> erk, this is right on the page allocator hotpath. Bad.

Yes, I know that it adds an overhead to allocation hot path, but I found
no other
place for such change. Do You have any suggestion where such change can
be applied
to avoid additional load on hot path?

>
> At the very least, we could code it so it is not quite so dreadfully
> inefficient:
>
> if (migratetype == MIGRATE_MOVABLE) {
> unsigned long nr_cma_free;
>
> nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
> if (nr_cma_free) {
> unsigned long nr_free;
>
> nr_free = zone_page_state(zone, NR_FREE_PAGES);
>
> if (nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
> migratetype = MIGRATE_CMA;
> }
> }
>
> but it still looks pretty bad.

Do You want me to resend such patch?

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center