Re: [PATCH v2 net-next] net: snmp: add statistics for tcp small queue check

[Menglong Dong]

On Mon, Nov 29, 2021 at 11:57 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Sun, 28 Nov 2021 14:01:02 +0800 menglong8.dong@xxxxxxxxx wrote:
> > Once tcp small queue check failed in tcp_small_queue_check(), the
> > throughput of tcp will be limited, and it's hard to distinguish
> > whether it is out of tcp congestion control.
> >
> > Add statistics of LINUX_MIB_TCPSMALLQUEUEFAILURE for this scene.
>
> Isn't this going to trigger all the time and alarm users because of the
> "Failure" in the TCPSmallQueueFailure name?  Isn't it perfectly fine
> for TCP to bake full TSQ amount of data and have it paced out onto the
> wire? What's your link speed?

Well, it's a little complex. In my case, there is a guest in kvm, and virtio_net
is used with napi_tx enabled.

With napi_tx enabled, skb won't be orphaned after it is passed to virtio_net,
until it is released. The point is that the sending interrupt of
virtio_net will be
turned off and the skb can't be released until the next net_rx interrupt comes.
So, wmem_alloc can't decrease on time, and the bandwidth is limited. When
this happens, the bandwidth can decrease from 500M to 10M.

In fact, this issue of uapi_tx is fixed in this commit:
https://lore.kernel.org/lkml/20210719144949.935298466@xxxxxxxxxxxxxxxxxxx/

I added this statistics to monitor the sending failure (may be called
sending delay)
caused by qdisc and net_device. When something happen, maybe users can
raise ‘/proc/sys/net/ipv4/tcp_pacing_ss_ratio’ to get better bandwidth.

Thanks!
Menglong Dong