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

On Tue, Nov 16, 2021 at 7:27 AM Jakub Kicinski [off-list ref] wrote:

On Tue, 16 Nov 2021 07:22:02 -0800 Eric Dumazet wrote:

quoted

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

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.
(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.