linux-kernel - Re: [PATCH v9 1/8] mm: Add per-cpu logic to page shuffling

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Date:   Tue, 10 Sep 2019 15:11:50 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     David Hildenbrand <david@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        mst@...hat.com, catalin.marinas@....com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        mhocko@...nel.org, linux-mm@...ck.org, akpm@...ux-foundation.org,
        will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        osalvador@...e.de
Cc:     yang.zhang.wz@...il.com, pagupta@...hat.com,
        konrad.wilk@...cle.com, nitesh@...hat.com, riel@...riel.com,
        lcapitulino@...hat.com, wei.w.wang@...el.com, aarcange@...hat.com,
        ying.huang@...el.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, fengguang.wu@...el.com,
        kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v9 1/8] mm: Add per-cpu logic to page shuffling

On Mon, 2019-09-09 at 10:14 +0200, David Hildenbrand wrote:
> On 07.09.19 19:25, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > 
> > Change the logic used to generate randomness in the suffle path so that we
> > can avoid cache line bouncing. The previous logic was sharing the offset
> > and entropy word between all CPUs. As such this can result in cache line
> > bouncing and will ultimately hurt performance when enabled.
> 
> So, usually we perform such changes if there is real evidence. Do you
> have any such performance numbers to back your claims?

I don't have any numbers. From what I can tell the impact is small enough
that this doesn't really have much impact.

With that being the case I can probably just drop this patch. I will
instead just use "rand & 1" in the 2nd patch to generate the return value
which was what was previously done in add_to_free_area_random.