linux-kernel - Re: [PATCH net-next v2] net: Modify mono_delivery_time with clockid_delivery

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Date: Wed, 28 Feb 2024 15:14:16 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Andrew Halaney <ahalaney@...hat.com>, 
 Martin KaFai Lau <martin.lau@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>
Cc: kernel@...cinc.com
Subject: Re: [PATCH net-next v2] net: Modify mono_delivery_time with
 clockid_delivery_time

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@...cinc.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

> > 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
 
> 
> 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.