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 07:21:33 +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 08/19/2015 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....
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