Date:   Mon, 20 Mar 2017 14:59:14 +0200
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH v2 net-next] mlx4: Better use of order-0 pages in RX path



On 15/03/2017 5:36 PM, Tariq Toukan wrote:
>
>
> On 14/03/2017 5:11 PM, Eric Dumazet wrote:
>> When adding order-0 pages allocations and page recycling in receive path,
>> I added issues on PowerPC, or more generally on arches with large pages.
>>
>> A GRO packet, aggregating 45 segments, ended up using 45 page frags
>> on 45 different pages. Before my changes we were very likely packing
>> up to 42 Ethernet frames per 64KB page.
>>
>> 1) At skb freeing time, all put_page() on the skb frags now touch 45
>>    different 'struct page' and this adds more cache line misses.
>>    Too bad that standard Ethernet MTU is so small :/
>>
>> 2) Using one order-0 page per ring slot consumes ~42 times more memory
>>    on PowerPC.
>>
>> 3) Allocating order-0 pages is very likely to use pages from very
>>    different locations, increasing TLB pressure on hosts with more
>>    than 256 GB of memory after days of uptime.
>>
>> This patch uses a refined strategy, addressing these points.
>>
>> We still use order-0 pages, but the page recyling technique is modified
>> so that we have better chances to lower number of pages containing the
>> frags for a given GRO skb (factor of 2 on x86, and 21 on PowerPC)
>>
>> Page allocations are split in two halves :
>> - One currently visible by the NIC for DMA operations.
>> - The other contains pages that already added to old skbs, put in
>>   a quarantine.
>>
>> When we receive a frame, we look at the oldest entry in the pool and
>> check if the page count is back to one, meaning old skbs/frags were
>> consumed and the page can be recycled.
>>
>> Page allocations are attempted using high order ones, trying
>> to lower TLB pressure. We remember in ring->rx_alloc_order the last
>> attempted
>> order and quickly decrement it in case of failures.
>> Then mlx4_en_recover_from_oom() called every 250 msec will attempt
>> to gradually restore rx_alloc_order to its optimal value.
>>
>> On x86, memory allocations stay the same. (One page per RX slot for
>> MTU=1500)
>> But on PowerPC, this patch considerably reduces the allocated memory.
>>
>> Performance gain on PowerPC is about 50% for a single TCP flow.
>>
>> On x86, I could not measure the difference, my test machine being
>> limited by the sender (33 Gbit per TCP flow).
>> 22 less cache line misses per 64 KB GRO packet is probably in the order
>> of 2 % or so.
>>
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> Cc: Tariq Toukan <tariqt@...lanox.com>
>> Cc: Saeed Mahameed <saeedm@...lanox.com>
>> Cc: Alexander Duyck <alexander.duyck@...il.com>
>> ---
>>  drivers/net/ethernet/mellanox/mlx4/en_rx.c   | 470
>> ++++++++++++++++-----------
>>  drivers/net/ethernet/mellanox/mlx4/en_tx.c   |  15 +-
>>  drivers/net/ethernet/mellanox/mlx4/mlx4_en.h |  54 ++-
>>  3 files changed, 317 insertions(+), 222 deletions(-)
>>
>
> Hi Eric,
>
> Thanks for your patch.
>
> I will do the XDP tests and complete the review, by tomorrow.

Hi Eric,

While testing XDP scenarios, I noticed a small degradation.
However, more importantly, I hit a kernel panic, see trace below.

I'll need time to debug this.
I will update about progress in debug and XDP testing.

If you want, I can do the re-submission myself once both issues are solved.

Thanks,
Tariq

Trace:
[  379.069292] BUG: Bad page state in process xdp2  pfn:fd8c04
[  379.075840] page:ffffea003f630100 count:-1 mapcount:0 mapping: 
   (null) index:0x0
[  379.085413] flags: 0x2fffff80000000()
[  379.089816] raw: 002fffff80000000 0000000000000000 0000000000000000 
ffffffffffffffff
[  379.098994] raw: dead000000000100 dead000000000200 0000000000000000 
0000000000000000
[  379.108154] page dumped because: nonzero _refcount
[  379.113793] Modules linked in: mlx4_en(OE) mlx4_ib ib_core 
mlx4_core(OE) netconsole nfsv3 nfs fscache dm_mirror dm_region_hash 
dm_log dm_mod sb_edac edac_core x86_pkg_temp_thermal coretemp 
i2c_diolan_u2c kvm iTCO_wdt iTCO_vendor_support lpc_ich ipmi_si 
irqbypass dcdbas ipmi_devintf crc32_pclmul mfd_core ghash_clmulni_intel 
pcspkr ipmi_msghandler sg wmi acpi_power_meter shpchp nfsd auth_rpcgss 
nfs_acl lockd grace sunrpc binfmt_misc ip_tables sr_mod cdrom sd_mod 
mlx5_core i2c_algo_bit drm_kms_helper tg3 syscopyarea sysfillrect 
sysimgblt fb_sys_fops ttm drm ahci libahci ptp megaraid_sas libata 
crc32c_intel i2c_core pps_core [last unloaded: mlx4_en]
[  379.179886] CPU: 38 PID: 6243 Comm: xdp2 Tainted: G           OE 
4.11.0-rc2+ #25
[  379.188846] Hardware name: Dell Inc. PowerEdge R730/0H21J3, BIOS 
1.5.4 10/002/2015
[  379.197814] Call Trace:
[  379.200838]  dump_stack+0x63/0x8c
[  379.204833]  bad_page+0xfe/0x11a
[  379.208728]  free_pages_check_bad+0x76/0x78
[  379.213688]  free_pcppages_bulk+0x4d5/0x510
[  379.218647]  free_hot_cold_page+0x258/0x280
[  379.228911]  __free_pages+0x25/0x30
[  379.233099]  mlx4_en_free_rx_buf.isra.23+0x79/0x110 [mlx4_en]
[  379.239811]  mlx4_en_deactivate_rx_ring+0xb2/0xd0 [mlx4_en]
[  379.246332]  mlx4_en_stop_port+0x4fc/0x7d0 [mlx4_en]
[  379.252166]  mlx4_xdp+0x373/0x3b0 [mlx4_en]
[  379.257126]  dev_change_xdp_fd+0x102/0x140
[  379.261993]  ? nla_parse+0xa3/0x100
[  379.266176]  do_setlink+0xc9c/0xcc0
[  379.270363]  ? nla_parse+0xa3/0x100
[  379.274547]  rtnl_setlink+0xbc/0x100
[  379.278828]  ? __enqueue_entity+0x60/0x70
[  379.283595]  rtnetlink_rcv_msg+0x95/0x220
[  379.288365]  ? __kmalloc_node_track_caller+0x214/0x280
[  379.294397]  ? __alloc_skb+0x7e/0x260
[  379.298774]  ? rtnl_newlink+0x830/0x830
[  379.303349]  netlink_rcv_skb+0xa7/0xc0
[  379.307825]  rtnetlink_rcv+0x28/0x30
[  379.312102]  netlink_unicast+0x15f/0x230
[  379.316771]  netlink_sendmsg+0x319/0x390
[  379.321441]  sock_sendmsg+0x38/0x50
[  379.325624]  SYSC_sendto+0xef/0x170
[  379.329808]  ? SYSC_bind+0xb0/0xe0
[  379.333895]  ? alloc_file+0x1b/0xc0
[  379.338077]  ? __fd_install+0x22/0xb0
[  379.342456]  ? sock_alloc_file+0x91/0x120
[  379.347314]  ? fd_install+0x25/0x30
[  379.351518]  SyS_sendto+0xe/0x10
[  379.355432]  entry_SYSCALL_64_fastpath+0x1a/0xa9
[  379.360901] RIP: 0033:0x7f824e6d0cad
[  379.365201] RSP: 002b:00007ffc75259a08 EFLAGS: 00000246 ORIG_RAX: 
000000000000002c
[  379.374198] RAX: ffffffffffffffda RBX: 00000000ffffffff RCX: 
00007f824e6d0cad
[  379.382481] RDX: 000000000000002c RSI: 00007ffc75259a20 RDI: 
0000000000000003
[  379.390746] RBP: 00000000ffffffff R08: 0000000000000000 R09: 
0000000000000000
[  379.399010] R10: 0000000000000000 R11: 0000000000000246 R12: 
0000000000000019
[  379.407273] R13: 0000000000000030 R14: 00007ffc7525b260 R15: 
00007ffc75273270
[  379.415539] Disabling lock debugging due to kernel taint


>
> Regards,
> Tariq

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