linux-kernel - Re: [PATCH] mm/page_alloc: give priority to free cma-pages from pcplist to buddy

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Date:   Sun, 24 Apr 2022 16:34:01 +1200
From:   Barry Song <21cnbao@...il.com>
To:     李培锋(wink) <lipeifeng@...o.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        robin.murphy@....com
Cc:     Andrew Morton <akpm@...ux-foundation.org>, peifeng55@...il.com,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        张诗明(Simon Zhang) 
        <zhangshiming@...o.com>
Subject: Re: [PATCH] mm/page_alloc: give priority to free cma-pages from
 pcplist to buddy

On Sun, Apr 24, 2022 at 3:28 PM <lipeifeng@...o.com> wrote:
>
> From: lipeifeng <lipeifeng@...o.com>
>
> Cma-pages will be fallback to movable pages in many scenarios.when cma
> pages are freed to pcplist, we give priority to free it from pcplist
> to buddy in order to  avoids cma-pages to be used as movable-pages soon
> if there is enough free-movable-pages, which saves more cma-pages in
> buddy to decrease pages migration when cma_alloc.
>
> Signed-off-by: lipeifeng <lipeifeng@...o.com>
> ---

+ Christoph, Marek, Robin as it is cma-related.


>  mm/page_alloc.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3589feb..69369ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3372,7 +3372,7 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone)
>  }
>
>  static void free_unref_page_commit(struct page *page, unsigned long pfn,
> -                                  int migratetype, unsigned int order)
> +                                  int migratetype, unsigned int order, bool fast_free)
>  {
>         struct zone *zone = page_zone(page);
>         struct per_cpu_pages *pcp;
> @@ -3382,7 +3382,10 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn,
>         __count_vm_event(PGFREE);
>         pcp = this_cpu_ptr(zone->per_cpu_pageset);
>         pindex = order_to_pindex(migratetype, order);
> -       list_add(&page->lru, &pcp->lists[pindex]);
> +       if (fast_free)
> +               list_add_tail(&page->lru, &pcp->lists[pindex]);
> +       else
> +               list_add(&page->lru, &pcp->lists[pindex]);

Ok. This is interesting, we used to have a separate cma pcp list but
now MIGRATE_CMA is an outsider so cma pages are placed in the
MIGRATE_MOVABLE list.

enum migratetype {
        MIGRATE_UNMOVABLE,
        MIGRATE_MOVABLE,
        MIGRATE_RECLAIMABLE,
        MIGRATE_PCPTYPES,       /* the number of types on the pcp lists */
        MIGRATE_HIGHATOMIC = MIGRATE_PCPTYPES,
#ifdef CONFIG_CMA
        ...
        MIGRATE_CMA,
#endif
#ifdef CONFIG_MEMORY_ISOLATION
        MIGRATE_ISOLATE,        /* can't allocate from here */
#endif
        MIGRATE_TYPES
};

#define NR_PCP_LISTS (MIGRATE_PCPTYPES * (PAGE_ALLOC_COSTLY_ORDER + 1
+ NR_PCP_THP))

>         pcp->count += 1 << order;
>         high = nr_pcp_high(pcp, zone);
>         if (pcp->count >= high) {
> @@ -3400,6 +3403,7 @@ void free_unref_page(struct page *page, unsigned int order)
>         unsigned long flags;
>         unsigned long pfn = page_to_pfn(page);
>         int migratetype;
> +       bool fast_free = false;
>
>         if (!free_unref_page_prepare(page, pfn, order))
>                 return;
> @@ -3419,9 +3423,15 @@ void free_unref_page(struct page *page, unsigned int order)
>                 }
>                 migratetype = MIGRATE_MOVABLE;
>         }
> +       /*
> +        * Give priority to free cma-pages to buddy in order to
> +        * decrease pages migration when cma_alloc.
> +        */
> +       if (migratetype == MIGRATE_CMA)
> +               fast_free = true;
>
>         local_lock_irqsave(&pagesets.lock, flags);
> -       free_unref_page_commit(page, pfn, migratetype, order);
> +       free_unref_page_commit(page, pfn, migratetype, order, fast_free);
>         local_unlock_irqrestore(&pagesets.lock, flags);
>  }
>
> @@ -3459,6 +3469,8 @@ void free_unref_page_list(struct list_head *list)
>
>         local_lock_irqsave(&pagesets.lock, flags);
>         list_for_each_entry_safe(page, next, list, lru) {
> +               bool fast_free = false;
> +
>                 pfn = page_private(page);
>                 set_page_private(page, 0);
>
> @@ -3467,11 +3479,19 @@ void free_unref_page_list(struct list_head *list)
>                  * to the MIGRATE_MOVABLE pcp list.
>                  */
>                 migratetype = get_pcppage_migratetype(page);
> +
> +               /*
> +                * Give priority to free cma-pages to buddy in order to
> +                * decrease pages migration when cma_alloc.
> +                */
> +               if (migratetype == MIGRATE_CMA)
> +                       fast_free = true;
> +
>                 if (unlikely(migratetype >= MIGRATE_PCPTYPES))
>                         migratetype = MIGRATE_MOVABLE;
>
>                 trace_mm_page_free_batched(page);
> -               free_unref_page_commit(page, pfn, migratetype, 0);
> +               free_unref_page_commit(page, pfn, migratetype, 0, fast_free);

i'd  call get_pcppage_migratetype() again in free_unref_page_commit()
rather than adding a parameter to increase a couple cross functions.

>
>                 /*
>                  * Guard against excessive IRQ disabled times when we get
> --
> 2.7.4
>

Thanks
Barry