netdev - Re: [RFC v2 3/3] iwlwifi: mvm: transfer the truesize to the last TSO segment

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Date:	Thu, 20 Aug 2015 07:40:08 +0000
From:	"Grumbach, Emmanuel" <emmanuel.grumbach@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"ido@...ery.com" <ido@...ery.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RFC v2 3/3] iwlwifi: mvm: transfer the truesize to the last
 TSO segment



On 08/20/2015 10:21 AM, Grumbach, Emmanuel wrote:
> 
> 
> On 08/19/2015 11:39 PM, Eric Dumazet wrote:
>> On Wed, 2015-08-19 at 19:17 +0000, Grumbach, Emmanuel wrote:
>>
>>> Hm.. how would net/core/tso.c avoid this?
>>
>> Because a driver using these helpers keep around the original LSO packet
>> and frees it normally at TX completion time.
>>
>>> I can't see anything related to truesize there.
>>> Note that this work since it is guaranteed that we release the skbs in
>>> order.
>>>
>>>>
>>>> (BTW TCP packets do not have sock_wfree as destructor but tcp_wfree(),
>>>> yet we want backpressure mostly for TCP stack (TCP Small Queues))
>>>>
>>>>
>>>
>>> I am not sure I follow here.
>>> You want me to test:
>>> if (skb_gso->destructor == tcp_wfree) ?
>>
>>
>> Yes.
>>
>> Look for example at tcp_gso_segment() (called from skb_gso_segment())
>>
>> copy_destructor = gso_skb->destructor == tcp_wfree;
>> ...
>>         /* Following permits TCP Small Queues to work well with GSO :
>>          * The callback to TCP stack will be called at the time last frag
>>          * is freed at TX completion, and not right now when gso_skb
>>          * is freed by GSO engine
>>          */
>>         if (copy_destructor) {
>>                 swap(gso_skb->sk, skb->sk);
>>                 swap(gso_skb->destructor, skb->destructor);
>>                 sum_truesize += skb->truesize;
>>                 atomic_add(sum_truesize - gso_skb->truesize,
>>                            &skb->sk->sk_wmem_alloc);
>>         }
>>
>>
>>>
>>> I checked that code using iperf and saw that I don't get into this if,
>>> but I (probably wrongly) assumed that other applications would set a
>>> flag on the socket (forgive my ignorance) that would make this if be taken.
>>
>> If you do not see skb->destructor == tcp_wfree, then something is
>> definitely wrong on your setup.
>>
> 
> tcp_wfree isn't exported. I can change that. It will be a challenge for
> backport though. Hm....
> 

But you seem to say that gso_skb->destructor *should* point to
tcp_wfree, so maybe testing gso_skb->destructor isn't NULL is good enough?
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