linux-kernel - Re: [PATCH 2/2] mm/page_alloc: Allow high-order pages to be stored on the per-cpu lists

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Message-ID: <88FCC7AA-FAAA-4B87-B382-50BD54B2886B@nvidia.com>
Date:   Wed, 09 Jun 2021 14:30:18 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] mm/page_alloc: Allow high-order pages to be stored on the per-cpu lists

On 3 Jun 2021, at 10:22, Mel Gorman wrote:

> The per-cpu page allocator (PCP) only stores order-0 pages. This means
> that all THP and "cheap" high-order allocations including SLUB contends
> on the zone->lock. This patch extends the PCP allocator to store THP and
> "cheap" high-order pages. Note that struct per_cpu_pages increases in
> size to 256 bytes (4 cache lines) on x86-64.
>
> Note that this is not necessarily a universal performance win because of
> how it is implemented. High-order pages can cause pcp->high to be exceeded
> prematurely for lower-orders so for example, a large number of THP pages
> being freed could release order-0 pages from the PCP lists. Hence, much
> depends on the allocation/free pattern as observed by a single CPU to
> determine if caching helps or hurts a particular workload.
>
> That said, basic performance testing passed. The following is a netperf
> UDP_STREAM test which hits the relevant patches as some of the network
> allocations are high-order.
>
> netperf-udp
>                                  5.13.0-rc2             5.13.0-rc2
>                            mm-pcpburst-v3r4   mm-pcphighorder-v1r7
> Hmean     send-64         261.46 (   0.00%)      266.30 *   1.85%*
> Hmean     send-128        516.35 (   0.00%)      536.78 *   3.96%*
> Hmean     send-256       1014.13 (   0.00%)     1034.63 *   2.02%*
> Hmean     send-1024      3907.65 (   0.00%)     4046.11 *   3.54%*
> Hmean     send-2048      7492.93 (   0.00%)     7754.85 *   3.50%*
> Hmean     send-3312     11410.04 (   0.00%)    11772.32 *   3.18%*
> Hmean     send-4096     13521.95 (   0.00%)    13912.34 *   2.89%*
> Hmean     send-8192     21660.50 (   0.00%)    22730.72 *   4.94%*
> Hmean     send-16384    31902.32 (   0.00%)    32637.50 *   2.30%*
>
> From a functional point of view, a patch like this is necessary to
> make bulk allocation of high-order pages work with similar performance
> to order-0 bulk allocations. The bulk allocator is not updated in this
> series as it would have to be determined by bulk allocation users how
> they want to track the order of pages allocated with the bulk allocator.
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  include/linux/mmzone.h |  20 +++++-
>  mm/internal.h          |   2 +-
>  mm/page_alloc.c        | 159 +++++++++++++++++++++++++++++------------
>  mm/swap.c              |   2 +-
>  4 files changed, 135 insertions(+), 48 deletions(-)
>

Hi Mel,

I am not able to boot my QEMU VM with v5.13-rc5-mmotm-2021-06-07-18-33.
git bisect points to this patch. The VM got stuck at “Booting from ROM…”.

My kernel config is attached and my qemu command is:

qemu-system-x86_64 -kernel ~/repos/linux-1gb-thp/arch/x86/boot/bzImage \
    -drive file=~/qemu-image/vm.qcow2,if=virtio \
    -append "nokaslr root=/dev/vda1 rw console=ttyS0 " \
    -pidfile vm.pid \
    -netdev user,id=mynet0,hostfwd=tcp::11022-:22 \
    -device virtio-net-pci,netdev=mynet0 \
    -m 16g -smp 6 -cpu host -enable-kvm -nographic \
    -machine hmat=on -object memory-backend-ram,size=8g,id=m0 \
    -object memory-backend-ram,size=8g,id=m1 \
    -numa node,memdev=m0,nodeid=0 -numa node,memdev=m1,nodeid=1

The attached config has THP disabled. The VM cannot boot with THP enabled,
either.

—
Best Regards,
Yan, Zi

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