netdev - Re: [PATCH net-next 2/3] tcp: implement coalescing on backlog queue

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Message-ID: <CANn89iJn7+C3He8RO3X9PS6k0rGH+mHgTMyc27xC3k+ge2JhnQ@mail.gmail.com>
Date:   Thu, 22 Nov 2018 10:16:04 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Jean-Louis Dupond <jean-louis@...ond.be>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 2/3] tcp: implement coalescing on backlog queue

On Thu, Nov 22, 2018 at 10:01 AM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Wed, Nov 21, 2018 at 12:52 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > In case GRO is not as efficient as it should be or disabled,
> > we might have a user thread trapped in __release_sock() while
> > softirq handler flood packets up to the point we have to drop.
> >
> > This patch balances work done from user thread and softirq,
> > to give more chances to __release_sock() to complete its work.
> >
> > This also helps if we receive many ACK packets, since GRO
> > does not aggregate them.
>
> Would this coalesce duplicate incoming ACK packets? Is there a risk
> that this would eliminate incoming dupacks needed for fast recovery in
> non-SACK connections? Perhaps pure ACKs should only be coalesced if
> the ACK field is different?

Yes, I was considering properly filtering SACK as a refinement later [1]
but you raise a valid point for alien stacks that are not yet using SACK :/

[1] This version of the patch will not aggregate sacks since the
memcmp() on tcp options would fail.

Neal can you double check if cake_ack_filter() does not have the issue
you just mentioned ?