Re: [PATCH v1 2/3] mm/memory_hotplug: don't shuffle complete zone when onlining memory

[Dan Williams]

On Tue, Jun 16, 2020 at 10:00 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2020 at 5:51 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > On Tue 16-06-20 13:52:12, David Hildenbrand wrote:
> > > Commit e900a918b098 ("mm: shuffle initial free memory to improve
> > > memory-side-cache utilization") introduced shuffling of free pages
> > > during system boot and whenever we online memory blocks.
> > >
> > > However, whenever we online memory blocks, all pages that will be
> > > exposed to the buddy end up getting freed via __free_one_page(). In the
> > > general case, we free these pages in MAX_ORDER - 1 chunks, which
> > > corresponds to the shuffle order.
> > >
> > > Inside __free_one_page(), we will already shuffle the newly onlined pages
> > > using "to_tail = shuffle_pick_tail();". Drop explicit zone shuffling on
> > > memory hotplug.

This was already explained in the initial patch submission. The
shuffle_pick_tail() shuffling at run time is only sufficient for
maintaining the shuffle. It's not sufficient for effectively
randomizing the free list.

See:

e900a918b098 mm: shuffle initial free memory to improve
memory-side-cache utilization

    This initial randomization can be undone over time so a follow-on patch is
    introduced to inject entropy on page free decisions.  It is reasonable to
    ask if the page free entropy is sufficient, but it is not enough due to
    the in-order initial freeing of pages.  At the start of that process
    putting page1 in front or behind page0 still keeps them close together,
    page2 is still near page1 and has a high chance of being adjacent.  As
    more pages are added ordering diversity improves, but there is still high
    page locality for the low address pages and this leads to no significant
    impact to the cache conflict rate.