Re: [PATCH RFC net-next] net: drop_monitor: Add debugfs support

[Wang Liang <wangliang74@huawei.com>]

在 2025/10/15 18:40, Eric Dumazet 写道:
> On Wed, Oct 15, 2025 at 2:51 AM Wang Liang <wangliang74@huawei.com> wrote:
>> This patch add debugfs interfaces for drop monitor. Similar to kmemleak, we
>> can use the monitor by below commands:
>>
>>    echo clear > /sys/kernel/debug/drop_monitor/trace
>>    echo start > /sys/kernel/debug/drop_monitor/trace
>>    echo stop  > /sys/kernel/debug/drop_monitor/trace
>>    cat /sys/kernel/debug/drop_monitor/trace
>>
>> The trace skb number limit can be set dynamically:
>>
>>    cat /sys/kernel/debug/drop_monitor/trace_limit
>>    echo 200 > /sys/kernel/debug/drop_monitor/trace_limit
>>
>> Compare to original netlink method, the callstack dump is supported. There
>> is a example for received udp packet with error checksum:
>>
>>    reason   : UDP_CSUM (11)
>>    pc       : udp_queue_rcv_one_skb+0x14b/0x350
>>    len      : 12
>>    protocol : 0x0800
>>    stack    :
>>      sk_skb_reason_drop+0x8f/0x120
>>      udp_queue_rcv_one_skb+0x14b/0x350
>>      udp_unicast_rcv_skb+0x71/0x90
>>      ip_protocol_deliver_rcu+0xa6/0x160
>>      ip_local_deliver_finish+0x90/0x100
>>      ip_sublist_rcv_finish+0x65/0x80
>>      ip_sublist_rcv+0x130/0x1c0
>>      ip_list_rcv+0xf7/0x130
>>      __netif_receive_skb_list_core+0x21d/0x240
>>      netif_receive_skb_list_internal+0x186/0x2b0
>>      napi_complete_done+0x78/0x190
>>      e1000_clean+0x27f/0x860
>>      __napi_poll+0x25/0x1e0
>>      net_rx_action+0x2ca/0x330
>>      handle_softirqs+0xbc/0x290
>>      irq_exit_rcu+0x90/0xb0
>>
>> It's more friendly to use and not need user application to cooperate.
>> Furthermore, it is easier to add new feature. We can add reason/ip/port
>> filter by debugfs parameters, like ftrace, rather than netlink msg.
> I do not understand the fascination with net/core/drop_monitor.c,
> which looks very old school to me,
> and misses all the features,  flexibility, scalability  that 'perf',
> eBPF tracing, bpftrace, .... have today.
>
> Adding  /sys/kernel/debug/drop_monitor/* is even more old school.
>
> Not mentioning the maintenance burden.
>
> For me the choice is easy :
>
> # CONFIG_NET_DROP_MONITOR is not set
>
> perf record -ag -e skb:kfree_skb sleep 1
>
> perf script # or perf report


Thank you for taking time to review this patch!

My initially thought was that the drop_monitor may cover more drop
positions (not just kfree_skb), show more skb info, filter skb by ip/port
debugfs parameter (not support now), and not need userspace tools.

Currently perf is indeed a better choice, adding debugfs is not necessary.

Thanks!