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: <20201005065648.GO4555@dhcp22.suse.cz>
Date:   Mon, 5 Oct 2020 08:56:48 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     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 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. If yes it would likely be better to
have this documented than relying on wild guess. If we do not have any
evidence then I would vote for simplicity first and go with
unconditional add_to_tail which would simply your patch a bit.

Maybe Vlastimil or Mel would have a better picture.

-- 
Michal Hocko
SUSE Labs