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

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Message-ID: <20200617064826.GI9499@dhcp22.suse.cz>
Date:   Wed, 17 Jun 2020 08:48:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v1 2/3] mm/memory_hotplug: don't shuffle complete zone
 when onlining memory

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@...el.com> wrote:
> >
> > On Tue, Jun 16, 2020 at 5:51 AM Michal Hocko <mhocko@...nel.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