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Date: Wed, 28 Feb 2024 19:09:27 -0500
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: "Abhishek Chauhan (ABC)" <quic_abchauha@quicinc.com>, 
 Willem de Bruijn <willemdebruijn.kernel@gmail.com>, 
 "David S. Miller" <davem@davemloft.net>, 
 Eric Dumazet <edumazet@google.com>, 
 Jakub Kicinski <kuba@kernel.org>, 
 Paolo Abeni <pabeni@redhat.com>, 
 netdev@vger.kernel.org, 
 linux-kernel@vger.kernel.org, 
 Andrew Halaney <ahalaney@redhat.com>, 
 Martin KaFai Lau <martin.lau@kernel.org>, 
 Martin KaFai Lau <martin.lau@linux.dev>
Cc: kernel@quicinc.com
Message-ID: <65dfcb37844e0_ba1f8294b7@willemb.c.googlers.com.notmuch>
In-Reply-To: <8a5d15aa-96e2-4d9c-9479-bf2ba8fb2a79@quicinc.com>
References: <20240228011219.1119105-1-quic_abchauha@quicinc.com>
 <65df56f6ba002_7162829435@willemb.c.googlers.com.notmuch>
 <f38efc6d-20af-4cc1-9b8a-5fcb676b2845@quicinc.com>
 <65df94185a2c1_b2ad829442@willemb.c.googlers.com.notmuch>
 <8a5d15aa-96e2-4d9c-9479-bf2ba8fb2a79@quicinc.com>
Subject: Re: [PATCH net-next v2] net: Modify mono_delivery_time with
 clockid_delivery_time
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Abhishek Chauhan (ABC) wrote:
> 
> 
> On 2/28/2024 12:14 PM, Willem de Bruijn wrote:
> > Abhishek Chauhan (ABC) wrote:
> >>
> >>
> >> On 2/28/2024 7:53 AM, Willem de Bruijn wrote:
> >>> Abhishek Chauhan wrote:
> >>>> Bridge driver today has no support to forward the userspace timestamp
> >>>> packets and ends up resetting the timestamp. ETF qdisc checks the
> >>>> packet coming from userspace and encounters to be 0 thereby dropping
> >>>> time sensitive packets. These changes will allow userspace timestamps
> >>>> packets to be forwarded from the bridge to NIC drivers.
> >>>>
> >>>> Existing functionality of mono_delivery_time is not altered here
> >>>> instead just extended with userspace tstamp support for bridge
> >>>> forwarding path.
> >>>>
> >>>> Signed-off-by: Abhishek Chauhan <quic_abchauha@quicinc.com>
> >>>> ---
> >>>> Changes since v1 
> >>>> - Changed the commit subject as i am modifying the mono_delivery_time 
> >>>>   bit with clockid_delivery_time.
> >>>> - Took care of suggestion mentioned by Willem to use the same bit for 
> >>>>   userspace delivery time as there are no conflicts between TCP and 
> >>>>   SCM_TXTIME, because explicit cmsg makes no sense for TCP and only
> >>>>   RAW and DGRAM sockets interprets it.
> >>>
> >>> The variable rename churn makes it hard to spot the functional
> >>> changes. Perhaps it makes sense just keep the variable name as is,
> >>> even though the "mono" is not always technically correct anymore.
> >>>
> >>   
> >>
> >> I think a better approach would be to keep the variable as ease and add
> >> comments and documentation in the header file of skbuff.h like 
> >> how i have done in this patch. The reason why i say this is
> >> a. We can avoid alot of code churn just to solve this basic problem of 
> >> propagating timestamp through forwarding bridge path 
> >> b. Re-use the same variable name and have better documentation 
> >> c. Complexity will be as minimal as possible.
> >>
> >> Let me know what you think. 
> > 
> > Agreed
> > 
> 
> Okay i will make the changes accordingly. 
> >>> Or else to split into two patches. One that renames the field.
> >>> And one that adds the new behavior of setting the bit for SO_TXTIME.
> >>>
> >>
> >> Regarding the sidenote. I dont see how they are using clock_id to determine 
> >> if the skb->tstamp is set in monotonic. Please correct me or point me to 
> >> the piece of code which is doing so.
> > 
> > That's really out of scope of this series anyway
> > 
> 
> Sounds good. Really appreciate your review and discussion on this topic. 
>  
> >>
> >> I hope the check against sock_flag is a better implementation as 
> >> it clearly stats and is inline with the implementation that the tstamp is 
> >> coming from userspace. 
> >> skb->mono_delivery_time = sock_flag(sk, SOCK_TXTIME);
> > 
> > Enabling the socket flag is not sufficient to configure a delivery
> > time on a packet. A transmit time must be communicated per packet
> > with cork->transmit_time. And on top of that, it is cheaper to test.
> 
> 
> So to re-use the same bit of mono_delivery_time. I want to set this bit 
> when user-space sets the timestamps using SCM_TXTIME. 
> Is it okay if i do the below when we make skb in ipv4/ipv6 and raw packets
> to ensure that bridge doesn't reset the packet tstamp or do you have a better 
> suggestion to set the bit so br_forward_finish does not reset the timestamp. 
> 
> skb->mono_delivery_time = sock_flag(sk, SOCK_TXTIME);

I already gave my suggestion.

The timestamp is passed using sockcm_cookie field transmit_time, which
is set from a control message of type SCM_TXTIME passed to sendmsg.

Right above where you wanted to add this check is where skb->tstamp is
initialized from cork->transmit_time, which got it from this field. So
clearly that is the indication that a transmit time is set. And the
field is hot in the cache.