netdev - Re: [PATCH net-next 17/20] tcp: defer skb freeing after socket lock is released

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Date:   Tue, 16 Nov 2021 13:45:18 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>
Subject: Re: [PATCH net-next 17/20] tcp: defer skb freeing after socket lock
 is released

On 11/16/21 9:46 AM, Eric Dumazet wrote:
> On Tue, Nov 16, 2021 at 7:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>
>> On Tue, 16 Nov 2021 07:22:02 -0800 Eric Dumazet wrote:
>>> Here is the perf top profile on cpu used by user thread doing the
>>> recvmsg(), at 96 Gbit/s
>>>
>>> We no longer see skb freeing related costs, but we still see costs of
>>> having to process the backlog.
>>>
>>>    81.06%  [kernel]       [k] copy_user_enhanced_fast_string
>>>      2.50%  [kernel]       [k] __skb_datagram_iter
>>>      2.25%  [kernel]       [k] _copy_to_iter
>>>      1.45%  [kernel]       [k] tcp_recvmsg_locked
>>>      1.39%  [kernel]       [k] tcp_rcv_established
>>
>> Huh, somehow I assumed your 4k MTU numbers were with zero-copy :o

I thought the same. :-)

>>
>> Out of curiosity - what's the softirq load with 4k? Do you have an
>> idea what the load is on the CPU consuming the data vs the softirq
>> processing with 1500B ?
> 
> On my testing host,
> 
> 4K MTU : processing ~2,600.000 packets per second in GRO and other parts
> use about 60% of the core in BH.

4kB or 4kB+hdr MTU? I ask because there is a subtle difference in the
size of the GRO packet which affects overall efficiency.

e.g., at 1500 MTU, 1448 MSS, a GRO packet has at most 45 segments for a
GRO size of 65212. At 4000 MTU, 3948 MSS, a GRO packet has at most 16
segments for a GRO packet size of 63220. I have noticed that 3300 MTU is
a bit of sweet spot with MLX5/ConnectX-5 at least - 20 segments and
65012 GRO packet without triggering nonlinear mode.


> (Some of this cost comes from a clang issue, and the csum_partial() one
> I was working on last week)
> NIC RX interrupts are firing about 25,000 times per second in this setup.
> 
> 1500 MTU : processing ~ 5,800,000 packets per second uses one core in
> BH (and also one core in recvmsg()),
> We stay in NAPI mode (no IRQ rearming)
> (That was with a TCP_STREAM run sustaining 70Gbit)
> 
> BH numbers also depend on IRQ coalescing parameters.
> 

What NIC do you use for testing?