Re: [PATCH v1 3/5] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype_isolate()

[Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>]

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.

-- 
Mel Gorman
SUSE Labs