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

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Message-ID: <20210825100618.687eedae@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 25 Aug 2021 10:06:18 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Zhongya Yan <yan2228598786@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.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 Wed, 25 Aug 2021 09:20:37 -0700 Eric Dumazet wrote:
> On Wed, Aug 25, 2021 at 9:04 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Wed, 25 Aug 2021 08:47:46 -0700 Eric Dumazet wrote:  
> > > 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  
> >
> > Yeah, the line number seems like a particularly bad idea. Hopefully
> > strings won't be problematic, given we can expect most serious users
> > to feed the tracepoints via BPF. enum would be more convenient there,
> > I'd think.
> >  
> > > 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.  
> >
> > I'm slightly surprised by this angle. Are there downstream kernels with
> > heavily modified TCP other than Google's?  
> 
> Not sure why Google is mentioned here ?
> Have you ever received a public report about TCP behavior in a Google kernel ?

That's a rhetorical question quite likely, but to be clear - what 
I meant is that Google is the main contributor to Linux TCP and has 
the expertise to make changes. I don't know of any others hence the
question.

> Over the years, we received hundreds of TCP bug reports on
> netdev@...r, where users claim to use  kernel version 4.19 (or other),
> when in fact they use 4.19.xxx
> It takes in general multiple emails exchange before we get a more
> realistic version number.
> Not to mention distro kernels, or even worse private kernels, which
> are not exactly easy to track for us upstream developers.

Right but for backports values come from original patch, enum or string.

I don't mean to dispute your preference tho, if you want strings,
strings it is.