From: Vlastimil Babka <vbabka@suse.cz>
To: Rik van Riel <riel@surriel.com>,
Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, Roman Gushchin <guro@fb.com>,
Qian Cai <cai@lca.pw>, Mel Gorman <mgorman@techsingularity.net>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations
Date: Wed, 11 Mar 2020 18:58:00 +0100 [thread overview]
Message-ID: <5ed7f24b-d21b-75a1-ff74-49a9e21a7b39@suse.cz> (raw)
In-Reply-To: <2f3e2cde7b94dfdb8e1f0532d1074e07ef675bc4.camel@surriel.com>
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?
>
next prev parent reply other threads:[~2020-03-11 17:58 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-06 20:01 [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations Rik van Riel
2020-03-07 22:38 ` Andrew Morton
2020-03-08 13:23 ` Rik van Riel
2020-03-11 17:58 ` Vlastimil Babka [this message]
2020-03-11 22:58 ` Roman Gushchin
2020-03-11 23:03 ` Vlastimil Babka
2020-03-11 23:21 ` Roman Gushchin
2020-03-11 8:51 ` Vlastimil Babka
2020-03-11 10:13 ` Joonsoo Kim
2020-03-11 17:41 ` Vlastimil Babka
2020-03-11 17:35 ` Roman Gushchin
2020-03-12 1:41 ` Joonsoo Kim
2020-03-12 2:39 ` Roman Gushchin
2020-03-12 8:56 ` Joonsoo Kim
2020-03-12 17:07 ` Roman Gushchin
2020-03-13 7:44 ` Joonsoo Kim
2020-04-02 2:13 ` Andrew Morton
2020-04-02 2:53 ` Roman Gushchin
2020-04-02 5:43 ` Joonsoo Kim
2020-04-02 19:42 ` Roman Gushchin
2020-04-03 4:34 ` Joonsoo Kim
2020-04-03 17:50 ` Roman Gushchin
2020-04-02 3:05 ` Roman Gushchin
2020-03-21 0:49 ` Minchan Kim
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