linux-kernel - Re: [PATCH v9 2/8] mm: Adjust shuffle code to allow for future coalescing

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Date:   Tue, 10 Sep 2019 07:48:41 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     virtio-dev@...ts.oasis-open.org, kvm list <kvm@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oscar Salvador <osalvador@...e.de>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Rik van Riel <riel@...riel.com>, lcapitulino@...hat.com,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v9 2/8] mm: Adjust shuffle code to allow for future coalescing

On Tue, Sep 10, 2019 at 5:20 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Sat 07-09-19 10:25:20, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> >
> > Move the head/tail adding logic out of the shuffle code and into the
> > __free_one_page function since ultimately that is where it is really
> > needed anyway. By doing this we should be able to reduce the overhead
> > and can consolidate all of the list addition bits in one spot.
>
> This changelog doesn't really explain why we want this. You are
> reshuffling the code, allright, but why do we want to reshuffle? Is the
> result readability a better code reuse or something else? Where
> does the claimed reduced overhead coming from?
>
> From a quick look buddy_merge_likely looks nicer than the code splat
> we have. Good.
>
> But then
>
> > Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
>
> [...]
>
> > -     if (is_shuffle_order(order))
> > -             add_to_free_area_random(page, &zone->free_area[order],
> > -                             migratetype);
> > +     area = &zone->free_area[order];
> > +     if (is_shuffle_order(order) ? shuffle_pick_tail() :
> > +         buddy_merge_likely(pfn, buddy_pfn, page, order))
>
> Ouch this is just awful don't you think?

Yeah. I am going to go with Kirill's suggestion and probably do
something more along the lines of:
       bool to_tail;
        ...
        if (is_shuffle_order(order))
                to_tail = shuffle_pick_tail();
       else
                to_tail = buddy_merge_likely(pfn, buddy_pfn, page, order);

        if (to_tail)
                add_to_free_area_tail(page, area, migratetype);
        else
                add_to_free_area(page, area, migratetype);