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Date:   Thu, 3 Dec 2020 11:03:21 -0800
From:   John Ousterhout <ouster@...stanford.edu>
To:     netdev@...r.kernel.org
Subject: GRO: can't force packet up stack immediately?

I recently upgraded my kernel module implementing the Homa transport
protocol from 4.15.18 to 5.4.80, and a GRO feature available in the
older version seems to have gone away in the newer version. In
particular, it used to be possible for a protocol's xxx_gro_receive
function to force a packet up the stack immediately by returning that
skb as the result of xxx_gro_receive. However, in the newer kernel
version, these packets simply get queued on napi->rx_list; the queue
doesn't get flushed up-stack until napi_complete_done is called or
gro_normal_batch packets accumulate. For Homa, this extra level of
queuing gets in the way.

Is there any way for a xxx_gro_receive function to force a packet (in
particular, one of those in the list passed as first argument to
xxx_gro_receive) up the protocol stack immediately? I suppose I could
set gro_normal_batch to 1, but that might interfere with other
protocols that really want the batching.

-John-

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