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Message-ID: <1cc1e3d1-4646-436c-92cb-f8584406bf88@suse.cz>
Date: Tue, 9 Apr 2024 08:23:20 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mgorman@...hsingularity.net>, Zi Yan <ziy@...dia.com>,
"Huang, Ying" <ying.huang@...el.com>, David Hildenbrand <david@...hat.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting
On 4/8/24 4:23 PM, Johannes Weiner wrote:
> On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
>> On 4/7/24 12:19 PM, Baolin Wang wrote:
>> > On 2024/3/21 02:02, Johannes Weiner wrote:
>> >>
>> >> + account_freepages(page, zone, 1 << order, migratetype);
>> >> +
>> >> while (order < MAX_PAGE_ORDER) {
>> >> - if (compaction_capture(capc, page, order, migratetype)) {
>> >> - __mod_zone_freepage_state(zone, -(1 << order),
>> >> - migratetype);
>> >> + int buddy_mt = migratetype;
>> >> +
>> >> + if (compaction_capture(capc, page, order, migratetype))
>> >> return;
>> >> - }
>> >
>> > IIUC, if the released page is captured by compaction, then the
>> > statistics for free pages should be correspondingly decreased,
>> > otherwise, there will be a slight regression for my thpcompact benchmark.
>> >
>> > thpcompact Percentage Faults Huge
>> > k6.9-rc2-base base + patch10 + 2 fixes
>> > Percentage huge-1 78.18 ( 0.00%) 71.92 ( -8.01%)
>> > Percentage huge-3 86.70 ( 0.00%) 86.07 ( -0.73%)
>> > Percentage huge-5 90.26 ( 0.00%) 78.02 ( -13.57%)
>> > Percentage huge-7 92.34 ( 0.00%) 78.67 ( -14.81%)
>> > Percentage huge-12 91.18 ( 0.00%) 81.04 ( -11.12%)
>> > Percentage huge-18 89.00 ( 0.00%) 79.57 ( -10.60%)
>> > Percentage huge-24 90.52 ( 0.00%) 80.07 ( -11.54%)
>> > Percentage huge-30 94.44 ( 0.00%) 96.28 ( 1.95%)
>> > Percentage huge-32 93.09 ( 0.00%) 99.39 ( 6.77%)
>> >
>> > I add below fix based on your fix 2, then the thpcompact Percentage
>> > looks good. How do you think for the fix?
>>
>> Yeah another well spotted, thanks. "slight regression" is an understatement,
>> this affects not just a "statistics" but very important counter
>> NR_FREE_PAGES which IIUC would eventually become larger than reality, make
>> the watermark checks false positive and result in depleted reserves etc etc.
>> Actually wondering why we're not seeing -next failures already (or maybe I
>> just haven't noticed).
>
> Good catch indeed.
>
> Trying to understand why I didn't notice this during testing, and I
> think it's because I had order-10 pageblocks in my config. There is
> this in compaction_capture():
>
> if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> return false;
>
> Most compaction is for order-9 THPs on movable blocks, so I didn't get
> much capturing in practice in order for that leak to be noticable.
>
> In earlier versions of the patches I had more aggressive capturing,
> but also did the accounting in add_page_to/del_page_from_freelist(),
> so capturing only steals what's already accounted to be off list.
>
> With the __add/__del and the consolidated account_freepages()
> optimization, compaction_capture() needs explicit accounting again.
>
>> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> > index 8330c5c2de6b..2facf844ef84 100644
>> > --- a/mm/page_alloc.c
>> > +++ b/mm/page_alloc.c
>> > @@ -805,8 +805,10 @@ static inline void __free_one_page(struct page *page,
>> > while (order < MAX_PAGE_ORDER) {
>> > int buddy_mt = migratetype;
>> >
>> > - if (compaction_capture(capc, page, order, migratetype))
>> > + if (compaction_capture(capc, page, order, migratetype)) {
>> > + account_freepages(zone, -(1 << order), migratetype);
>> > return;
>> > + }
>> >
>> > buddy = find_buddy_page_pfn(page, pfn, order, &buddy_pfn);
>> > if (!buddy)
>> >
>> > With my fix, the THP percentage looks better:
>> > k6.9-rc2-base base + patch10 + 2 fixes +
>> > my fix
>> > Percentage huge-1 78.18 ( 0.00%) 82.83 ( 5.94%)
>> > Percentage huge-3 86.70 ( 0.00%) 93.47 ( 7.81%)
>> > Percentage huge-5 90.26 ( 0.00%) 94.73 ( 4.95%)
>> > Percentage huge-7 92.34 ( 0.00%) 95.22 ( 3.12%)
>> > Percentage huge-12 91.18 ( 0.00%) 92.40 ( 1.34%)
>> > Percentage huge-18 89.00 ( 0.00%) 85.39 ( -4.06%)
>> > Percentage huge-24 90.52 ( 0.00%) 94.70 ( 4.61%)
>> > Percentage huge-30 94.44 ( 0.00%) 97.00 ( 2.71%)
>> > Percentage huge-32 93.09 ( 0.00%) 92.87 ( -0.24%)
>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
> With fixed indentation:
Maybe Baolin could resend the finalized 2 fixups in a more ready-to-pick form?
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 70f82635f650..96815e3c22f2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -805,8 +805,10 @@ static inline void __free_one_page(struct page *page,
> while (order < MAX_PAGE_ORDER) {
> int buddy_mt = migratetype;
>
> - if (compaction_capture(capc, page, order, migratetype))
> + if (compaction_capture(capc, page, order, migratetype)) {
> + account_freepages(zone, -(1 << order), migratetype);
> return;
> + }
>
> buddy = find_buddy_page_pfn(page, pfn, order, &buddy_pfn);
> if (!buddy)
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