netdev - Re: [PATCH] net: tcp_drop adds `reason` parameter for tracing v2

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Date:   Wed, 01 Sep 2021 10:36:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Brendan Gregg <brendan.d.gregg@...il.com>
CC:     Eric Dumazet <edumazet@...gle.com>,
        Zhongya Yan <yan2228598786@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, hengqi.chen@...il.com,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH] net: tcp_drop adds `reason` parameter for tracing v2

On Thu, 26 Aug 2021 15:13:07 +1000
Brendan Gregg <brendan.d.gregg@...il.com> wrote:

> On Thu, Aug 26, 2021 at 1:20 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Wed, 25 Aug 2021 08:47:46 -0700
> > Eric Dumazet <edumazet@...gle.com> wrote:
> >  
> > > > @@ -5703,15 +5700,15 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
> > > >                         TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
> > > >                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
> > > >                 tcp_send_challenge_ack(sk, skb);
> > > > -               goto discard;
> > > > +               tcp_drop(sk, skb, TCP_DROP_MASK(__LINE__, TCP_VALIDATE_INCOMING));  
> > >
> > > I'd rather use a string. So that we can more easily identify _why_ the
> > > packet was drop, without looking at the source code
> > > of the exact kernel version to locate line number 1057
> > >
> > > You can be sure that we will get reports in the future from users of
> > > heavily modified kernels.
> > > Having to download a git tree, or apply semi-private patches is a no go.
> > >
> > > If you really want to include __FILE__ and __LINE__, these both can be
> > > stringified and included in the report, with the help of macros.  
> >
> > I agree the __LINE__ is pointless, but if this has a tracepoint
> > involved, then you can simply enable the stacktrace trigger to it and
> > it will save a stack trace in the ring buffer for you.
> >
> >    echo stacktrace > /sys/kernel/tracing/events/tcp/tcp_drop/trigger
> >
> > And when the event triggers it will record a stack trace. You can also
> > even add a filter to do it only for specific reasons.
> >
> >    echo 'stacktrace if reason == 1' > /sys/kernel/tracing/events/tcp/tcp_drop/trigger
> >
> > And it even works for flags:
> >
> >    echo 'stacktrace if reason & 0xa' > /sys/kernel/tracing/events/tcp/tcp_drop/trigger
> >
> > Which gives another reason to use an enum over a string.  
> 
> You can't do string comparisons? The more string support Ftrace has,
> the more convenient they will be. Using bpftrace as an example of
> convenience and showing drop frequency counted by human-readable
> reason and stack trace:

Yes, you can (and pretty much always had this ability), but having
flags is usually makes it easier (and faster).

You can have 'stacktrace if reason ~ "*string*"' which will match
anything with "string" in it.

My main argument against strings is more of the space they take up in
the ring buffer than the ability to filter.

-- Steve

-- 
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