netdev - Re: [PATCH v2 net-next 2/5] net-smnp: reorganize SNMP fast path variables

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Message-ID: <CANn89iKfXxaLr0b-rp0_+X7QY82pK21zeLCVjqxNipfKkwOnDg@mail.gmail.com>
Date: Tue, 17 Oct 2023 21:15:24 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Coco Li <lixiaoyan@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Neal Cardwell <ncardwell@...gle.com>, Mubashir Adnan Qureshi <mubashirq@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, Chao Wu <wwchao@...gle.com>, 
	Wei Wang <weiwan@...gle.com>, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v2 net-next 2/5] net-smnp: reorganize SNMP fast path variables

On Tue, Oct 17, 2023 at 9:10 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Tue, Oct 17, 2023 at 08:10:21PM +0200, Eric Dumazet wrote:
> > On Tue, Oct 17, 2023 at 3:57 AM Andrew Lunn <andrew@...n.ch> wrote:
> > >
> > > On Tue, Oct 17, 2023 at 01:47:13AM +0000, Coco Li wrote:
> > > > From: Chao Wu <wwchao@...gle.com>
> > > >
> > > > Reorganize fast path variables on tx-txrx-rx order.
> > > > Fast path cacheline ends afer LINUX_MIB_DELAYEDACKLOCKED.
> > > > There are only read-write variables here.
> > > >
> > > > Below data generated with pahole on x86 architecture.
> > > >
> > > > Fast path variables span cache lines before change: 12
> > > > Fast path variables span cache lines after change: 2
> > >
> > > As i pointed out for the first version, this is a UAPI file.
> > >
> > > Please could you add some justification that this does not cause any
> > > UAPI changes. Will old user space binaries still work after this?
> > >
> > > Thanks
> > >         Andrew
> >
> > I do not think the particular order is really UAPI. Not sure why they
> > were pushed in uapi in the first place.
> >
> > Kernel exports these counters with a leading line with the names of the metrics.
> >
> > We already in the past added fields and nothing broke.
> >
> > So the answer is : user space binaries not ignoring the names of the
> > metrics will work as before.
> >
> > nstat is one of the standard binary.
>
> This is the sort of thing which i think should be in the commit
> message. It makes it clear somebody has thought about this, and they
> think the risk is minimal. Without such a comment, somebody will ask
> if changing to a uapi file is safe.

Sure, although we never said such a thing in prior changes.

Perhaps add a big comment in the file itself, instead of repeating it
on future commit changelogs ?