[ovs-dev] [PATCH v7] datapath: Add Stateless TCP Tunneling protocol.

Jesse Gross jesse at nicira.com
Thu Apr 23 03:50:31 UTC 2015


On Wed, Apr 22, 2015 at 8:37 PM, Pravin Shelar <pshelar at nicira.com> wrote:
> On Wed, Apr 22, 2015 at 8:29 PM, Jesse Gross <jesse at nicira.com> wrote:
>> On Wed, Apr 22, 2015 at 8:22 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>> On Wed, Apr 22, 2015 at 7:48 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>> On Wed, Apr 22, 2015 at 3:38 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>> On Wed, Apr 22, 2015 at 3:24 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>> On Wed, Apr 22, 2015 at 3:19 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>> On Wed, Apr 22, 2015 at 11:31 AM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>> On Wed, Apr 22, 2015 at 11:09 AM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>> On Wed, Apr 22, 2015 at 10:52 AM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>> On Wed, Apr 22, 2015 at 10:03 AM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>> On Mon, Apr 20, 2015 at 9:54 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>>>> On Mon, Apr 20, 2015 at 7:33 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 5:56 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:54 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:36 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 1:29 PM, Pravin B Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>>>>>>>> diff --git a/datapath/linux/compat/stt.c b/datapath/linux/compat/stt.c
>>>>>>>>>>>>>>>>> new file mode 100644
>>>>>>>>>>>>>>>>> index 0000000..209bf1a
>>>>>>>>>>>>>>>>> --- /dev/null
>>>>>>>>>>>>>>>>> +++ b/datapath/linux/compat/stt.c
>>>>>>>>>>>>>>>>> +static void update_headers(struct sk_buff *skb, bool head,
>>>>>>>>>>>>>>>>> +                              unsigned int l4_offset, unsigned int hdr_len,
>>>>>>>>>>>>>>>>> +                              bool ipv4, u32 tcp_seq)
>>>>>>>>>>>>>>>> [...]
>>>>>>>>>>>>>>>>> +       skb->truesize = SKB_TRUESIZE(skb_end_offset(skb)) + skb->data_len;
>>>>>>>>>>>>>>>>> +}
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> I wonder if there are any possible edge cases with resetting truesize
>>>>>>>>>>>>>>>> where the packet is still in someone's transmit queue (such as if we
>>>>>>>>>>>>>>>> are looping back packet). Do we need to orphan it first?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> ok, I will orphan it in update_headers.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Just to clarify - I was mostly just thinking aloud on orphaning it.
>>>>>>>>>>>>>> I'm not totally sure if that is the right thing to do or if this is
>>>>>>>>>>>>>> the right place to do it. I'm not sure what the conceptual
>>>>>>>>>>>>>> justification would be for it and it could potentially result in the
>>>>>>>>>>>>>> sender's buffers not being properly limited. Perhaps not resetting the
>>>>>>>>>>>>>> truesize is the right thing too...
>>>>>>>>>>>>>>
>>>>>>>>>>>>> I have seen warning msg if we do no keep truesize update along with
>>>>>>>>>>>>> changes to skb.
>>>>>>>>>>>>
>>>>>>>>>>>> Hmm, interesting, what is the warning? I don't think that I have seen
>>>>>>>>>>>> that before.
>>>>>>>>>>>
>>>>>>>>>>> Actually skb_try_coalesce() is updating it correctly. so there no need
>>>>>>>>>>> to change truesize anymore. I will update patch accordingly.
>>>>>>>>>>
>>>>>>>>>> That's much nicer. I also checked and other receive side code (like
>>>>>>>>>> TCP input) doesn't worry about the case where a local sender may still
>>>>>>>>>> be accounting for the packet since any type of loopback device does
>>>>>>>>>> call skb_orphan() in some form.
>>>>>>>>>>
>>>>>>>>>> I hate to bring this up but what about on transmit? In cases where we
>>>>>>>>>> merge or split skbs (skb_try_coalesce() and normalize_frag_list()
>>>>>>>>>> respectively) we do track the truesize for correctly for the result
>>>>>>>>>> but the individual pieces might not have the right destructors or
>>>>>>>>>> might not have their truesize updated for the destructor they do have.
>>>>>>>>>
>>>>>>>>> How about about we update merged skb stats (len, data_len, truesize)
>>>>>>>>> according to *delta_truesize we get from skb_try_coalesce() and then
>>>>>>>>> free the skb?
>>>>>>>>
>>>>>>>> I think that would work for the skb_try_coalesce() case (although I
>>>>>>>> would only worry about truesize, not the lengths). For
>>>>>>>> normalize_frag_list() I think we would either have to add a destructor
>>>>>>>> or not update truesize. I'm not sure that the condition where we have
>>>>>>>> frag_lists and a destructor actually ever happens in practice though.
>>>>>>>
>>>>>>> I am not sure why do we need a destructor for normalize_frag_list
>>>>>>> changes. Even with the changes in the function truesize is consistent
>>>>>>> for given skb memory usage.
>>>>>>
>>>>>> If you have a packet with a frag_list, all of the memory for the
>>>>>> individual elements will be accounted for in the truesize in the top
>>>>>> level skb. This skb could also be accounted to some socket and have a
>>>>>> destructor. When we break apart the list, we remove the truesize from
>>>>>> the head because the memory is now part of individual packets.
>>>>>> However, the destructor is presumably only on the head and so only its
>>>>>> memory will be removed from the socket accounting when it is freed but
>>>>>> not each of the other skbs that came from it.
>>>>>
>>>>> ok. In that case we can not have our own destructor since there is one
>>>>> already (I am not sure if we can use skb->cb to restore original). not
>>>>> changing true size can complicate skb coalesce, since it does update
>>>>> truesize. Easy option would be orphan skb if we are going to coalesce
>>>>> its fragments.
>>>>
>>>> I'm not sure that we need our own destructor. What do you think about
>>>> just replicating the original destructor onto each of the newly
>>>> generated skbs?
>>>>
>>> In that case we assume there is no state associated with the skb, that
>>> might not be always true.
>>
>> What state do you mean? If you mean a destructor on the individual
>> skbs, I think that is already true because only the top level
>> destructor will get called when the original skb is freed.
>
> If destructor is replicated on the individual skbs then it will be
> called for each of those skbs when it is freed.

Right; if we've correctly apportioned truesize among the individual
skbs isn't that the goal?



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