netdev - Re: [PATCH net-next v4] net: Re-use and set mono_delivery

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Message-ID: <66ad9e5b-0126-476e-bf0f-6a33f446c976@quicinc.com>
Date: Mon, 18 Mar 2024 12:02:09 -0700
From: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>, <kernel@...cinc.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>, bpf
	<bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Alexei
 Starovoitov" <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH net-next v4] net: Re-use and set mono_delivery_time bit
 for userspace tstamp packets



On 3/14/2024 3:29 PM, Abhishek Chauhan (ABC) wrote:
> 
> 
> On 3/14/2024 2:48 PM, Martin KaFai Lau wrote:
>> On 3/14/24 1:53 PM, Abhishek Chauhan (ABC) wrote:
>>>>> The bpf_convert_tstamp_{read,write} and the helper bpf_skb_set_tstamp need to be
>>>>> changed to handle the new "user_delivery_time" bit anyway, e.g.
>>>>> bpf_skb_set_tstamp(BPF_SKB_TSTAMP_DELIVERY_MONO) needs to clear the
>>>>> "user_delivery_time" bit.
>>>>>
>>>>> I think the "struct inet_frag_queue" also needs a new "user_delivery_time"
>>>>> field. "mono_delivery_time" is already in there.
>>
>> [ ... ]
>>

Martin, Do we really need to add user_delivery_time as part of inet_frag_queue struct? I was wondering why is this required since we are using tstamp_type:2 to 
distinguish between timestamp anyway .

Let me know what you think ? 

>> I would think the first step is to revert this patch. I don't think much of the current patch can be reused.
>>
>>> 1. I will raise one patch to introduce rename mono_delivery_time to
>>> tstamp_type
>>
>> Right, I expect something like this:
>>
>> struct sk_buff {
>>         /* ... */
>> -            __u8                    mono_delivery_time:1;
>> +        __u8            tstamp_type:1;
>>         /* ... */
>> };
>>
> 
> Okay ,This should be straight-forward. 
> 
>>> 2. I will introduce setting of userspace timestamp type as the second bit
>>> whem transmit_time is set.
>>
>> I expect the second patch should be introducing the enum first
>>
>> enum skb_tstamp_type {
>>     SKB_TSTAMP_TYPE_RX_REAL = 0, /* A RX (receive) time in real */
>>     SKB_TSTAMP_TYPE_TX_MONO = 1, /* A TX (delivery) time in mono */
>> };
>>
>> and start doing "skb->tstamp_type = SKB_TSTAMP_TYPE_TX_MONO;" instead of
>> "skb->tstamp_type = 1;"
>>
>> and the same for "skb->tstamp_type = SKB_TSTAMP_TYPE_RX_REAL;" instead of
>> "skb->tstamp_type = 0;"
>>
>>
>> This one I am not sure but probably need to change the skb_set_delivery_time() function signature also:
>>
>> static inline void skb_set_delivery_time(struct sk_buff *skb, ktime_t kt,
>> -                                        bool mono)
>> +                     enum skb_tstamp_type tstamp_type)
>>
> This should be straight-forward as well 
> 
>> The third patch is to change tstamp_type from 1 bit to 2 bits and add SKB_TSTAMP_TYPE_TX_USER.
>>
>> struct sk_buff {
>>         /* ... */
>> -        __u8            tstamp_type:1;
>> +        __u8            tstamp_type:2;
>>         /* ... */
>> };
>>
>> enum skb_tstamp_type {
>>     SKB_TSTAMP_TYPE_RX_REAL = 0,    /* A RX (receive) time in real */
>>     SKB_TSTAMP_TYPE_TX_MONO = 1,    /* A TX (delivery) time in mono */
>> +    SKB_TSTAMP_TYPE_TX_USER = 2,    /* A TX (delivery) time and its clock
>>                      * is in skb->sk->sk_clockid.
>>                      */
>>                
>> };
>>
>> This will shift a bit out of the byte where tstamp_type lives. It should be the "inner_protocol_type" bit by my hand count. Please check if it is directly used in bpf instruction (filter.c). As far as I look, it is not, so should be fine. Some details about bpf instruction accessible skb bit field here: https://lore.kernel.org/all/20230321014115.997841-1-kuba@kernel.org/
> This is where i would need thorough reviews from you and Willem as my area of expertise is limited to part of network stack and BPF is not one of them. 
> But i have plan on this and i know how to do it. 
> 
> Expect patches to be arriving to your inboxes next week, as we have a long weekend in Qualcomm 
> Fingers crossed :) 
> 
>>
>>
>>> 3. This will be a first step to make the design scalable.
>>> 4. Tomorrow if we have more timestamp to support, upstream community has to do is
>>> update the enum and increase the bitfield from 2=>3 and so on.
>>>
>>> I need help from Martin to test the patch which renames the mono_delivery_time
>>> to tstamp_type (Which i feel should be straight forward as the value of the bit is 1)
>>
>> The bpf change is not a no-op rename of mono_delivery_time. It needs to take care of the new bit added to the tstamp_type. Please see the previous email (and I also left it in the beginning of this email).
>>
>> Thus, you need to compile the selftests/bpf/ and run it to verify the changes when handling the new bit. The Documentation/bpf/bpf_devel_QA.rst has the howto details. You probably only need the newer llvm (newer gcc should work also as bpf CI has been using it) and the newer pahole. I can definitely help if there is issue in running the test_progs in selftests/bpf or you have question on making the changes in filter.c. To run the test: "./test_progs -t tc_redirect/tc_redirect_dtime"
>>