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

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Date:   Sun, 08 Mar 2020 09:23:06 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Roman Gushchin <guro@...com>,
        Qian Cai <cai@....pw>, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH]  mm,page_alloc,cma: conditionally prefer cma pageblocks
 for movable allocations

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@...riel.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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