Re: [RFC net 1/2] af_packet: retire TPACKET_V3 blocks without waiting for copiers

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jingxuan Xie wrote:
> From: jingxuanxie <jingxuanxie@deeproute.ai>
> 
> TPACKET_V3 waits for a ring-wide read lock before retiring a block,
> while holding sk_receive_queue.lock. A copier keeps the read lock
> after reserving a frame and dropping the queue lock. On PREEMPT_RT,
> preempting that copier can therefore delay another packet path or
> the retirement timer until the copier runs again.
> 
> This was investigated after an approximately 355 ms block-retirement
> wait in a real PREEMPT_RT deployment. A QEMU ARM64 SMP PREEMPT_RT
> reproducer uses test-only instrumentation to delay one outgoing
> copier after reservation, with a second sender forcing retirement.
> The unmodified retirement logic blocks the second sender. The
> instrumentation is not part of this series.
> 
> Track FREE, ACTIVE, SEALED and USER state per block in private
> metadata. Reserve and complete copies under the receive queue lock,
> retaining the original block reference across the unlocked copy.
> Seal a full or expired block and try the immediately following block
> without waiting. Publish sealed blocks in reservation order once
> their copies have completed. Do not reuse a sealed block merely
> because its mmap status still reads TP_STATUS_KERNEL.
> 
> Limit each locked publication batch to eight blocks and continue
> ready backlogs on an unbound worker. Stop the timer and worker before
> freeing metadata. Check readiness using the last published block,
> and validate virtio headers before reserving a frame so an invalid
> header cannot strand an unfinished reservation.
> 
> Tested against that reproducer with both full-block and timer
> retirement: B completes while A remains delayed, blocks are not
> published early, and sequence numbers, lengths and payload match.
> An uninstrumented QEMU guest also passes psock_tpacket and
> psock_fanout. The accompanying KUnit suite covers the state helpers.
> 
> Keep the mmap layout and userspace status protocol unchanged. Ring
> exhaustion can still drop capture copies; a stalled copier no longer
> has to block retirement or copying into another available block.
> 
> The block producer index must cover the full unsigned block count,
> matching the independent publication index across large rings.
> 
> Assisted-by: LLM
> Fixes: 632ca50f2cbd ("af_packet: TPACKET_V3: replace busy-wait loop")
> Signed-off-by: Jingxuan Xie <jingxuanxie@deeproute.ai>
> ---
>  net/packet/af_packet.c | 319 +++++++++++++++++++++++++----------------
>  net/packet/internal.h  |  27 +++-

This is a huge change. Try to solve the problem with a minimal fix.

The patch is doing more in one patch than needed. Such as moving when
virtio_net_hdr_to_skb is called. Probably for good reason but then that is
a separate change.

The main purpose can hopefully be achieved with less code churn too.

Try to avoid re-acquiring sk_receive_queue.lock, a regression. Batching
adds complexity and is probably not needed. system_dfl_wq runs as
SCHED_NORMAL, so this may introduce a new priority inversion.

Also: whitespace corruption, a confidential ("top secret") footer.