netdev - Re: [net-next PATCH V5] qdisc: bulk dequeue support for qdiscs with TCQ_F

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Date:	Tue, 30 Sep 2014 14:25:09 +0200
From:	Florian Westphal <fw@...len.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Dave Taht <dave.taht@...il.com>, toke@...e.dk
Subject: Re: [net-next PATCH V5] qdisc: bulk dequeue support for qdiscs with
 TCQ_F_ONETXQUEUE

Eric Dumazet <eric.dumazet@...il.com> wrote:
> >  	struct sk_buff *skb = q->gso_skb;
> > @@ -70,10 +106,17 @@ static inline struct sk_buff *dequeue_skb(struct Qdisc *q)
> >  		} else
> >  			skb = NULL;
> >  	} else {
> > -		if (!(q->flags & TCQ_F_ONETXQUEUE) || !netif_xmit_frozen_or_stopped(txq)) {
> > +		if (!(q->flags & TCQ_F_ONETXQUEUE) ||
> > +		    !netif_xmit_frozen_or_stopped(txq)) {
> > +			int bytelimit = qdisc_avail_bulklimit(txq);
> > +
> >  			skb = q->dequeue(q);
> > -			if (skb)
> > +			if (skb) {
> > +				bytelimit -= skb->len;
> >  				skb = validate_xmit_skb(skb, qdisc_dev(q));
> > +			}
> > +			if (skb && qdisc_may_bulk(q))
> > +				skb = try_bulk_dequeue_skb(q, skb, bytelimit);
> >  		}
> >  	}
> 
> Okay...
> 
> In my opinion, you guys missed some opportunities :
> 
> 1) Ideally the algo should try to call validate_xmit_skb() outside of
> the locks (qdisc lock or device lock), because eventually doing checksum
> computation or full segmentation should allow other cpus doing enqueues.

You're right, but this is not related to this patch specifically.
But sure, we can look into moving validate_xmit_skb calls outside of the
qdisc root locked sections.

> 2) TSO support. Have you ideas how to perform this ?

Yes, the idea was to just remove the skb_is_gso() test and limit
dql_avail() result to e.g. 128k to avoid a '8-fat-gso-skbs-in-a-row' scenario.

Did you have any other ideas in mind?

Thanks,
Florian
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