netdev - Re: [Patch net-next v3 2/3] net_sched: plug in qdisc ops change_tx_queue

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Message-ID: <CAM_iQpVKHgeYgqWTA9pjEzr641uuVEON7XtHNE-0U6nz3B95RA@mail.gmail.com>
Date:   Sun, 28 Jan 2018 18:31:10 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch net-next v3 2/3] net_sched: plug in qdisc ops change_tx_queue_len

On Fri, Jan 26, 2018 at 6:16 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
>
> This drops all packets in the queue. I don't think tweaking the queue
> length did this previously - did it?

No, because previously only enqueue reads the value.

> If not this change might surprise some people.

It is hard to say which behavior is better, it depends on what you
expect:

1) If you want to tx_queue_len change immediately takes affects,
it should drop those in queue too, maybe not all, but at least when
we shrinking the queue length, we should drop those beyond limit.

2) If you want to tx_queue_len change takes affects after all pending
packets are pushed out. This is literally the old behavior, so at some
moments, the number of packets in queue could be larger than the
new tx_queue_len.

I don't see which one is obviously better than the other one,
therefore it is hard to say which one people really expect.