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

[Vlastimil Babka <vbabka@suse.cz>]

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.

Otherwise I agree this is a reasonable solution until CMA is rewritten.

> 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?
>