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From:   Dongli Zhang <dongli.zhang@oracle.com>
To:     netdev@vger.kernel.org, bpf@vger.kernel.org
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Subject: [PATCH v2 0/3] tun/tap: use kfree_skb_reason() to trace dropped skb
Date:   Sat, 19 Feb 2022 11:12:43 -0800
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The commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") has
introduced the kfree_skb_reason() to help track the reason.

The tun and tap are commonly used as virtio-net/vhost-net backend. This is to
use kfree_skb_reason() to trace the dropped skb for those two drivers. 

Changed since v1:

I have renamed many of the reasons since v1. I make them as generic as possible
so that they can be re-used by core networking and drivers.

- SKB_DROP_REASON_SKB_CSUM

This is used whenever there is checksum error with sk_buff.

- SKB_DROP_REASON_SKB_COPY_DATA

The kernel may (zero) copy the data to or from sk_buff, e.g.,
zerocopy_sg_from_iter(), skb_copy_datagram_from_iter() and
skb_orphan_frags_rx(). This reason is for the copy related error.

- SKB_DROP_REASON_SKB_GSO_SEG

Any error reported when GSO processing the sk_buff. It is frequent to process
sk_buff gso data and we introduce a new reason to handle that.
	
- SKB_DROP_REASON_SKB_PULL
- SKB_DROP_REASON_SKB_TRIM

It is frequent to pull to sk_buff data or trim the sk_buff data.

- SKB_DROP_REASON_DEV_HDR

Any driver may report error if there is any error in the metadata on the DMA
ring buffer.

- SKB_DROP_REASON_DEV_READY

The device is not ready/online or initialized to receive data.

- SKB_DROP_REASON_DEV_FILTER

David Ahern suggested SKB_DROP_REASON_TAP_FILTER. I changed from 'TAP' to 'DEV'
to make it more generic.

- SKB_DROP_REASON_FULL_RING

Suggested by Eric Dumazet.

- SKB_DROP_REASON_BPF_FILTER

Dropped by ebpf filter


This is the output for TUN device.

# cat /sys/kernel/debug/tracing/trace_pipe
          <idle>-0       [020] b.s3.  2317.035738: kfree_skb: skbaddr=00000000feed21ec protocol=4 location=000000003e0b7d3d reason: FULL_RING
          arping-7078    [028] b..1.  2317.065261: kfree_skb: skbaddr=000000003409d3c3 protocol=2054 location=000000003e0b7d3d reason: FULL_RING
      vhost-5409-5422    [001] b..1.  2317.818647: kfree_skb: skbaddr=0000000004b8c03a protocol=2054 location=000000003e0b7d3d reason: FULL_RING
          arping-7078    [028] b..1.  2318.065336: kfree_skb: skbaddr=000000003409d3c3 protocol=2054 location=000000003e0b7d3d reason: FULL_RING
      vhost-5409-5422    [001] b..1.  2318.845315: kfree_skb: skbaddr=0000000004b8c03a protocol=2054 location=000000003e0b7d3d reason: FULL_RING

This is the output for TAP device.

# cat /sys/kernel/debug/tracing/trace_pipe 
    kworker/19:1-243     [019] ..s1.   922.639393: kfree_skb: skbaddr=0000000045a155b0 protocol=2054 location=00000000f88b4996 reason: FULL_RING
    kworker/19:1-243     [019] ..s1.   923.663398: kfree_skb: skbaddr=00000000ff4e054b protocol=2054 location=00000000f88b4996 reason: FULL_RING
    kworker/19:1-243     [019] ..s1.   924.687351: kfree_skb: skbaddr=00000000ff4e054b protocol=2054 location=00000000f88b4996 reason: FULL_RING
    kworker/19:1-243     [019] ..s1.   925.711294: kfree_skb: skbaddr=00000000fb75a48b protocol=2054 location=00000000f88b4996 reason: FULL_RING


 drivers/net/tap.c          | 30 ++++++++++++++++++++++--------
 drivers/net/tun.c          | 38 ++++++++++++++++++++++++++++++--------
 include/linux/skbuff.h     | 16 ++++++++++++++++
 include/trace/events/skb.h | 10 ++++++++++


Please let me know if there is any suggestion on the definition of reasons.

Thank you very much!

Dongli Zhang