[MPTCP] Re: [PATCH] mptcp: attempt skb coalescing when moving skbs to mptcp rx queue

[MPTCP] Re: [PATCH] mptcp: attempt skb coalescing when moving skbs to mptcp rx queue

[Paolo Abeni]

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On Mon, 2020-05-25 at 18:02 +0200, Florian Westphal wrote:
> We can try to coalesce the skbs we take from the subflows rx queue
> with the tail of the mptcp rx queue.
> 
> If successful, the skb head can be discarded early.
> 
> We can also free the skb extensions, we do not access them after
> this so no need to hold on to them.
> 
> Only caveat: do not built too fat skbs. They are only free'd once
> userspace has read all data.

To share IRC thoughts here:

It looks like TCP allows skb to grow above GSO_MAX_SIZE and frees them
only when user-space read all the data. Could we relax this constraint,
too?

skb_try_coalesce() will still somehow bound the skb size to
MAX_SKB_FRAGS * <something [PAGE_SIZE ?!?]>

Cheers,

Paolo

[MPTCP] Re: [PATCH] mptcp: attempt skb coalescing when moving skbs to mptcp rx queue

[Florian Westphal]

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Paolo Abeni <pabeni(a)redhat.com> wrote:
> On Mon, 2020-05-25 at 18:02 +0200, Florian Westphal wrote:
> > We can try to coalesce the skbs we take from the subflows rx queue
> > with the tail of the mptcp rx queue.
> > 
> > If successful, the skb head can be discarded early.
> > 
> > We can also free the skb extensions, we do not access them after
> > this so no need to hold on to them.
> > 
> > Only caveat: do not built too fat skbs. They are only free'd once
> > userspace has read all data.
> 
> To share IRC thoughts here:
> 
> It looks like TCP allows skb to grow above GSO_MAX_SIZE and frees them
> only when user-space read all the data. Could we relax this constraint,
> too?

Yes, but I saw skbs of up ~320kb Truesize, which i considered too
much.  I will remove the comment and the contraint in v2.