netdev - Re: [PATCH net-next 3/3] tcp: implement head drops in backlog queue

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Message-ID: <CANn89iKzg-Osj35=LtCMuA+A3hjEWojAThjvEhbX68+zxCL2_A@mail.gmail.com>
Date:   Wed, 21 Nov 2018 15:52:56 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Yuchung Cheng <ycheng@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Jean-Louis Dupond <jean-louis@...ond.be>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next 3/3] tcp: implement head drops in backlog queue

On Wed, Nov 21, 2018 at 3:47 PM Yuchung Cheng <ycheng@...gle.com> wrote:
>
> On Wed, Nov 21, 2018 at 2:47 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> >
> > On 11/21/2018 02:40 PM, Yuchung Cheng wrote:
> >> On Wed, Nov 21, 2018 at 9:52 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> >>> Under high stress, and if GRO or coalescing does not help,
> >>> we better make room in backlog queue to be able to keep latest
> >>> packet coming.
> >>>
> >>> This generally helps fast recovery, given that we often receive
> >>> packets in order.
> >>
> >> I like the benefit of fast recovery but I am a bit leery about head
> >> drop causing HoLB on large read, while tail drops can be repaired by
> >> RACK and TLP already. Hmm -
> >
> > This is very different pattern here.
> >
> > We have a train of packets coming, the last packet is not a TLP probe...
> >
> > Consider this train coming from an old stack without burst control nor pacing.
> >
> > This patch guarantees last packet will be processed, and either :
> >
> > 1) We are a receiver, we will send a SACK. Sender will typically start recovery
> >
> > 2) We are a sender, we will process the most recent ACK sent by the receiver.
> >
>
> Sure on the sender it's universally good.
>
> On the receiver side my scenario was not the last packet being TLP.
> AFAIU the new design will first try coalesce the incoming skb to the
> tail one then exit. Otherwise it's queued to the back with an
> additional 64KB space credit. This patch checks the space w/o the
> extra credit and drops the head skb. If the head skb has been
> coalesced, we might end dropping the first big chunk that may need a
> few rounds of fast recovery to repair. But I am likely to
> misunderstand the patch :-)

This situation happens only when we are largely over committing
the memory, due to bad skb->len/skb->truesize ratio.

Think about MTU=9000 and some drivers allocating 16 KB of memory per frame,
even if the packet has 1500 bytes in it.

>
> Would it make sense to check the space first before the coalesce
> operation, and drop just enough bytes of the head to make room?

This is basically what the patch does, the while loop breaks when we have freed
just enough skbs.