linux-kernel - Re: [PATCH v1 3/5] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype

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Message-ID: <d466322e-054c-9303-5eb3-833dce410651@redhat.com>
Date:   Mon, 5 Oct 2020 11:11:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-hyperv@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Mike Rapoport <rppt@...nel.org>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v1 3/5] mm/page_alloc: always move pages to the tail of
 the freelist in unset_migratetype_isolate()

On 05.10.20 10:20, Mel Gorman wrote:
> On Mon, Oct 05, 2020 at 08:56:48AM +0200, Michal Hocko wrote:
>> On Fri 02-10-20 17:20:09, David Hildenbrand wrote:
>>> On 02.10.20 15:24, Michal Hocko wrote:
>>>> On Mon 28-09-20 20:21:08, David Hildenbrand wrote:
>>>>> Page isolation doesn't actually touch the pages, it simply isolates
>>>>> pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
>>>>>
>>>>> We already place pages to the tail of the freelists when undoing
>>>>> isolation via __putback_isolated_page(), let's do it in any case
>>>>> (e.g., if order <= pageblock_order) and document the behavior.
>>>>>
>>>>> Add a "to_tail" parameter to move_freepages_block() but introduce a
>>>>> a new move_to_free_list_tail() - similar to add_to_free_list_tail().
>>>>>
>>>>> This change results in all pages getting onlined via online_pages() to
>>>>> be placed to the tail of the freelist.
>>>>
>>>> Is there anything preventing to do this unconditionally? Or in other
>>>> words is any of the existing callers of move_freepages_block benefiting
>>>> from adding to the head?
>>>
>>> 1. mm/page_isolation.c:set_migratetype_isolate()
>>>
>>> We move stuff to the MIGRATE_ISOLATE list, we don't care about the order
>>> there.
>>>
>>> 2. steal_suitable_fallback():
>>>
>>> I don't think we care too much about the order when already stealing
>>> pageblocks ... and the freelist is empty I guess?
>>>
>>> 3. reserve_highatomic_pageblock()/unreserve_highatomic_pageblock()
>>>
>>> Not sure if we really care.
>>
>> Honestly, I have no idea. I can imagine that some atomic high order
>> workloads (e.g. in net) might benefit from cache line hot pages but I am
>> not sure this is really observable.
> 
> The highatomic reserve is more concerned that about the allocation
> succeeding than it is about cache hotness.
> 

Thanks Mel and Michal. I'll simplify this patch then - and if it turns
out to be an actual problem, we can change that one instance, adding a
proper comment.

Thanks!

-- 
Thanks,

David / dhildenb