netdev - Re: [PATCH net-next v2] net: Print hashed skb addresses for all net and qdisc events

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Date:   Mon, 27 Jun 2022 13:49:26 -0600
From:   "Subash Abhinov Kasiviswanathan (KS)" <quic_subashab@...cinc.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>,
        <quic_jzenner@...cinc.com>, <cong.wang@...edance.com>,
        <qitao.xu@...edance.com>, <bigeasy@...utronix.de>,
        <rostedt@...dmis.org>, <mingo@...hat.com>,
        Sean Tranchetti <quic_stranche@...cinc.com>
Subject: Re: [PATCH net-next v2] net: Print hashed skb addresses for all net
 and qdisc events



On 6/27/2022 1:36 PM, Cong Wang wrote:
> On Fri, Jun 24, 2022 at 03:34:25PM -0600, Subash Abhinov Kasiviswanathan wrote:
>> The following commits added support for printing the real address-
>> 65875073eddd ("net: use %px to print skb address in trace_netif_receive_skb")
>> 70713dddf3d2 ("net_sched: introduce tracepoint trace_qdisc_enqueue()")
>> 851f36e40962 ("net_sched: use %px to print skb address in trace_qdisc_dequeue()")
>>
>> However, tracing the packet traversal shows a mix of hashes and real
>> addresses. Pasting a sample trace for reference-
>>
>> ping-14249   [002] .....  3424.046612: netif_rx_entry: dev=lo napi_id=0x3 queue_mapping=0
>> skbaddr=00000000dcbed83e vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x0000 protocol=0x0800
>> ip_summed=0 hash=0x00000000 l4_hash=0 len=84 data_len=0 truesize=768 mac_header_valid=1
>> mac_header=-14 nr_frags=0 gso_size=0 gso_type=0x0
>> ping-14249   [002] .....  3424.046615: netif_rx: dev=lo skbaddr=ffffff888e5d1000 len=84
>>
>> Switch the trace print formats to %p for all the events to have a
>> consistent format of printing the hashed addresses in all cases.
>>
> 
> This is obscured...
> 
> What exactly is the inconsistency here? Both are apparently hex, from
> user's point of view. The only difference is one is an apparently
> invalid kernel address, the other is not. This difference only matters
> when you try to dereference it, but I don't think you should do it here,
> this is not a raw tracepoint at all. You can always use raw tracepoints
> to dereference it without even bothering whatever we print.
> 
> Thanks.

Matching skbs addresses (in a particular format) helps to track the 
packet traversal timings / delays in processing.