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

[Michal Hocko <mhocko@kernel.org>]

On Tue 16-06-20 10:03:31, Dan Williams wrote:
> On Tue, Jun 16, 2020 at 10:00 AM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > On Tue, Jun 16, 2020 at 5:51 AM Michal Hocko <mhocko@kernel.org> 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.

Yes, the "randomness" of the added memory will be lower. But is this
observable for hotplug scenarios? Is memory hotplug for the normal
memory really a thing in setups which use RAM as a cache?

While I do agree that the code wise the shuffling per online operation
doesn't really have any overhead really but it would be really great to
know whether it matters at all.
-- 
Michal Hocko
SUSE Labs