netdev - Re: [PATCH net-next v3 1/2] net-timestamp: filter out report when setting SOF_TIMESTAMPING

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Message-ID: <CAL+tcoAxVG7xpjXG+gyqoJjspn16F3Nfq5-8BoyVqO13qFm5xg@mail.gmail.com>
Date: Sat, 31 Aug 2024 23:00:27 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, willemb@...gle.com, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v3 1/2] net-timestamp: filter out report when
 setting SOF_TIMESTAMPING_SOFTWARE

On Sat, Aug 31, 2024 at 10:50 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > introduce a new flag SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER in the
> > receive path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> > out rx timestamp report, especially after a process turns on
> > netstamp_needed_key which can time stamp every incoming skb.
> >
> > Previously, We found out if an application starts first which turns on
> > netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
> > could also get rx timestamp. Now we handle this case by introducing this
> > new flag without breaking users.
> >
> > In this way, we have two kinds of combination:
> > 1. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_RX_SOFTWARE, it
> > will surely allow users to get the rx timestamp report.
> > 2. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER
> > while the skb is timestamped, it will stop reporting the rx timestamp.
>
> The one point this commit does not explain is why a process would want
> to request software timestamp reporting, but not receive software
> timestamp generation. The only use I see is when the application does
> request SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE.

Yes, you're right.

>
> > Another thing about errqueue in this patch I have a few words to say:
> > In this case, we need to handle the egress path carefully, or else
> > reporting the tx timestamp will fail. Egress path and ingress path will
> > finally call sock_recv_timestamp(). We have to distinguish them.
> > Errqueue is a good indicator to reflect the flow direction.
>
> Good find on skb_is_err_queue rather than open-coding PACKET_OUTGOING.
>
> > Suggested-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>

Thanks for your review:)

>
> > ---
> > 1. Willem suggested this alternative way to solve the issue, so I
> > added his Suggested-by tag here. Thanks!
> > ---
> >  Documentation/networking/timestamping.rst | 12 ++++++++++++
> >  include/uapi/linux/net_tstamp.h           |  3 ++-
> >  net/core/sock.c                           |  4 ++++
> >  net/ethtool/common.c                      |  1 +
> >  net/ipv4/tcp.c                            |  7 +++++--
> >  net/socket.c                              |  5 ++++-
> >  6 files changed, 28 insertions(+), 4 deletions(-)
> >
> > diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> > index 5e93cd71f99f..ef2a334d373e 100644
> > --- a/Documentation/networking/timestamping.rst
> > +++ b/Documentation/networking/timestamping.rst
> > @@ -266,6 +266,18 @@ SOF_TIMESTAMPING_OPT_TX_SWHW:
> >    two separate messages will be looped to the socket's error queue,
> >    each containing just one timestamp.
> >
> > +SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER:
> > +  Used in the receive software timestamp. Enabling the flag along with
> > +  SOF_TIMESTAMPING_SOFTWARE will not report the rx timestamp to the
> > +  userspace so that it can filter out the case where one process starts
> > +  first which turns on netstamp_needed_key through setting generation
> > +  flags like SOF_TIMESTAMPING_RX_SOFTWARE, then another one only passing
> > +  SOF_TIMESTAMPING_SOFTWARE report flag could also get the rx timestamp.
> > +
> > +  SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER prevents the application from
> > +  being influenced by others and let the application finally choose
> > +  whether to report the timestamp in the receive path or not.
> > +
>
> nit, so no need to revise, but for next time:
>
> Documentation should describe the current state, not a history of
> changes (log), so wording like "finally" does not belong.

I learned it, and will bear it in my mind:)

Thanks,
Jason