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

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Message-ID: <aac8e0db-1bd7-0991-e2e1-58a5580387c0@redhat.com>
Date:   Mon, 22 Jun 2020 09:27:45 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.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 20.06.20 03:41, Dan Williams wrote:
> 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?

Hmm, mixed feelings. I (working for a distributor :) ) prefer fixes tags
for actual BUGs, as described in

Documentation/process/submitting-patches.rst: "If your patch fixes a bug
in a specific commit, e.g. you found an issue using ``git bisect``,
please use the 'Fixes:' tag with the first 12 characters" ...

So unless there are strong feelings, I'll not add a fixes tag (although
I agree, that it should have been contained in the original commit).

>> 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/

Huh, what went wrong there :) Thanks!

-- 
Thanks,

David / dhildenb