linux-kernel - Re: [PATCH v2 2/3] mm/memory_hotplug: document why shuffle

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Date:   Fri, 19 Jun 2020 18:41:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2 2/3] mm/memory_hotplug: document why shuffle_zone() is relevant

On Fri, Jun 19, 2020 at 6:00 AM David Hildenbrand <david@...hat.com> wrote:
>
> It's not completely obvious why we have to shuffle the complete zone, as
> some sort of shuffling is already performed when onlining pages via
> __free_one_page(), placing MAX_ORDER-1 pages either to the head or the tail
> of the freelist. Let's document why we have to shuffle the complete zone
> when exposing larger, contiguous physical memory areas to the buddy.
>

How about?

Fixes: e900a918b098 ("mm: shuffle initial free memory to improve
memory-side-cache utilization")

...just like Patch1 since that original commit was missing the proper
commentary in the code?


> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory_hotplug.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 9b34e03e730a4..a0d81d404823d 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -822,6 +822,14 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
>         zone->zone_pgdat->node_present_pages += onlined_pages;
>         pgdat_resize_unlock(zone->zone_pgdat, &flags);
>
> +       /*
> +        * When exposing larger, physically contiguous memory areas to the
> +        * buddy, shuffling in the buddy (when freeing onlined pages, putting
> +        * them either to the head or the tail of the freelist) is only helpful
> +        * for mainining the shuffle, but not for creating the initial shuffle.

s/mainining/maintaining/

> +        * Shuffle the whole zone to make sure the just onlined pages are
> +        * properly distributed across the whole freelist.
> +        */
>         shuffle_zone(zone);
>
>         node_states_set_node(nid, &arg);

Other than the above minor fixups you can add:

Acked-by: Dan Williams <dan.j.williams@...el.com>