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

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Date:   Tue, 10 Sep 2019 14:11:30 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     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,
        linux-mm@...ck.org, akpm@...ux-foundation.org, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, osalvador@...e.de,
        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 09-09-19 08:11:36, Alexander Duyck wrote:
> 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'll have to go rerun the test to get the exact numbers. The reason this
> came up is that my original test was spanning NUMA nodes and that made
> this more expensive as a result since the memory was both not local to the
> CPU and was being updated by multiple sockets.

What was the pattern of page freeing in your testing? I am wondering
because order 0 pages should be prevailing and those usually go via pcp
lists so they do not get shuffled unless the batch is full IIRC.
-- 
Michal Hocko
SUSE Labs