linux-kernel - Re: [PATCH RFC 1/4] net: skb: use line number to trace dropped skb

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Date:   Thu, 3 Feb 2022 09:13:52 -0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mingo@...hat.com, davem@...emloft.net, kuba@...nel.org,
        edumazet@...gle.com, yoshfuji@...ux-ipv6.org, dsahern@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        imagedong@...cent.com, joao.m.martins@...cle.com,
        joe.jin@...cle.com, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH RFC 1/4] net: skb: use line number to trace dropped skb


Hi David,

On 2/3/22 7:48 AM, David Ahern wrote:
> On 2/3/22 8:37 AM, Dongli Zhang wrote:
>> Sometimes the kernel may not directly call kfree_skb() to drop the sk_buff.
>> Instead, it "goto drop" and call kfree_skb() at 'drop'. This make it
>> difficult to track the reason that the sk_buff is dropped.
>>
>> The commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") has
>> introduced the kfree_skb_reason() to help track the reason. However, we may
>> need to define many reasons for each driver/subsystem.
>>
>> To avoid introducing so many new reasons, this is to use line number
>> ("__LINE__") to trace where the sk_buff is dropped. As a result, the reason
>> will be generated automatically.
>>
> 
> I don't agree with this approach. It is only marginally better than the
> old kfree_skb that only gave the instruction pointer. That tells you the
> function that dropped the packet, but not why the packet is dropped.
> Adding the line number only makes users have to consult the source code.
> 
> When I watch drop monitor for kfree_skb I want to know *why* the packet
> was dropped, not the line number in the source code. e.g., dropmon
> showing OTHERHOST means too many packets are sent to this host (e.g.,
> hypervisor) that do not belong to the host or the VMs running on it, or
> packets have invalid checksum (IP, TCP, UDP). Usable information by
> everyone, not just someone with access to the source code for that
> specific kernel.
> 
Thank you very much for the suggestion!

I will not follow this approach. I will introduce new reasons to TUN and TAP
drivers.

Dongli Zhang