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

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Message-ID: <8a5d15aa-96e2-4d9c-9479-bf2ba8fb2a79@quicinc.com>
Date: Wed, 28 Feb 2024 13:53:43 -0800
From: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
To: 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



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@...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
> 

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);