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

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Date:	Wed, 19 Aug 2015 13:39:22 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Grumbach, Emmanuel" <emmanuel.grumbach@...el.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 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.



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