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overlay/batch: ship never-grown slots as plain writes

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A slot that stays single-segment is byte-identical to the packet it
was seeded from, but flushSlot re-emitted it via WriteGSO with a
seeded pseudo-sum, forcing the kernel to software-checksum up to
~1400B that arrived with a perfectly valid checksum. Keep the borrowed
seed packet on the slot (valid until Flush per the Commit contract)
and emit it through the plain DATA_VALID path when numSeg is still 1
at flush time. appendPayload and mergeSlots only touch hdrBuf once
numSeg >= 2, so the raw bytes are pristine whenever the fast path
fires. This is every non-coalesced TCP/UDP packet: request/response
flows, many-flow fan-in, and each run's leftover tail.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp
This commit is contained in:
JackDoan
2026-07-29 17:31:29 -05:00
parent 69cf816f80
commit 2190900107
4 changed files with 214 additions and 163 deletions
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overlay/batch/tcp_coalesce.go
+12 -3
View File
@@ -37,7 +37,11 @@ const tcpCoalesceHdrCap = 100
// The caller (listenOut) must keep those buffers alive until Flush.
type coalesceSlot struct {
passthrough bool
rawPkt []byte // borrowed when passthrough
// rawPkt is borrowed: the whole packet for passthrough slots, the seed
// packet for coalesce slots. A coalesce slot that never grows past one
// segment is emitted from rawPkt so its original (already valid) L4
// checksum ships DATA_VALID instead of making the kernel recompute it.
rawPkt []byte
fk flowKey
hdrBuf [tcpCoalesceHdrCap]byte
@@ -235,7 +239,12 @@ func (c *TCPCoalescer) Flush() error {
var first error
for _, s := range c.slots {
var err error
if s.passthrough {
if s.passthrough || s.numSeg == 1 {
// A slot that never grew (nor absorbed a merge) is byte-identical
// to the packet it was seeded from; ship the original so its valid
// checksum rides the DATA_VALID path instead of paying a kernel
// software csum. appendPayload and mergeSlots only touch hdrBuf
// once numSeg >= 2, so rawPkt is still pristine here.
_, err = c.w.Write(s.rawPkt)
} else {
err = c.flushSlot(s)
@@ -268,7 +277,7 @@ func (c *TCPCoalescer) seed(pkt []byte, info parsedTCP) {
}
s := c.take()
s.passthrough = false
s.rawPkt = nil
s.rawPkt = pkt // kept for the numSeg==1 fast path in Flush
copy(s.hdrBuf[:], pkt[:info.hdrLen])
s.hdrLen = info.hdrLen
s.ipHdrLen = info.ipHdrLen