Messages in this thread | | | From | Eric Dumazet <> | Date | Tue, 30 Nov 2021 07:55:50 -0800 | Subject | Re: [PATCH v2 net-next] net: snmp: add statistics for tcp small queue check |
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On Tue, Nov 30, 2021 at 7:23 AM Jakub Kicinski <kuba@kernel.org> wrote: > > On Tue, 30 Nov 2021 22:36:59 +0800 Menglong Dong wrote: > > On Mon, Nov 29, 2021 at 11:57 PM Jakub Kicinski <kuba@kernel.org> wrote: > > > > > > On Sun, 28 Nov 2021 14:01:02 +0800 menglong8.dong@gmail.com 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@linuxfoundation.org/ > > > > 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. > > Sounds very second-order and particular to a buggy driver :/ > Let's see what Eric says but I vote revert.
I did some tests yesterday, using one high speed TCP_STREAM (~90Gbit), and got plenty of increments when using pfifo_fast qdisc. Yet seed was nominal (bottleneck is the copyout() cost at receiver)
I got few counter increments when qdisc is fq as expected (because of commit c73e5807e4f6fc6d "tcp: tsq: no longer use limit_output_bytes for paced flows")
So this new SNMP counter is not a proxy for the kind of problems that a buggy driver would trigger.
I also suggest we revert this patch.
Thanks !
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