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

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Date:   Sat, 2 Jul 2022 21:36:19 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Subash Abhinov Kasiviswanathan (KS)" <quic_subashab@...cinc.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        quic_jzenner@...cinc.com, "Cong Wang ." <cong.wang@...edance.com>,
        Qitao Xu <qitao.xu@...edance.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <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 Mon, Jun 27, 2022 at 12:49 PM Subash Abhinov Kasiviswanathan (KS)
<quic_subashab@...cinc.com> wrote:
>
>
>
> 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.

So... how didn't you notice the duplicated addresses with hashed ones?
It is 100% reproducible to see duplicates with hashed ones.

Thanks.