Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations

[Roman Gushchin <guro@fb.com>]

On Wed, Mar 11, 2020 at 06:58:00PM +0100, Vlastimil Babka wrote:
> On 3/8/20 2:23 PM, Rik van Riel wrote:
> > 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?
> 
> I agree it should be in the noise. But please do put it behind CONFIG_CMA
> #ifdef. My x86_64 desktop distro kernel doesn't have CONFIG_CMA. Even if this is
> effectively no-op with __rmqueue_cma_fallback() returning NULL immediately, I
> think the compiler cannot eliminate the two zone_page_state()'s which are
> atomic_long_read(), even if it's just ultimately READ_ONCE() here, that's a
> volatile cast which means elimination not possible AFAIK? Other architectures
> might be even more involved.

I agree.

Andrew,
can you, please, squash the following diff into the patch?

Thank you!

--

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d9067b75dcb..bc65931b3901 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2767,6 +2767,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
 {
        struct page *page;
 
+#ifdef CONFIG_CMA
        /*
         * Balance movable allocations between regular and CMA areas by
         * allocating from CMA when over half of the zone's free memory
@@ -2779,6 +2780,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
                if (page)
                        return page;
        }
+#endif
 retry:
        page = __rmqueue_smallest(zone, order, migratetype);
        if (unlikely(!page)) {