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SubjectRe: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso_bug
On 2014/08/19 16:10, Michael Chan wrote:
> On Tue, 2014-08-19 at 11:52 -0700, Benjamin Poirier wrote:
> > @@ -7838,11 +7838,14 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
> > struct netdev_queue *txq, struct sk_buff *skb)
> > {
> > struct sk_buff *segs, *nskb;
> > - u32 frag_cnt_est = skb_shinfo(skb)->gso_segs * 3;
> >
> > - /* Estimate the number of fragments in the worst case */
> > - if (unlikely(tg3_tx_avail(tnapi) <= frag_cnt_est)) {
> > + if (unlikely(tg3_tx_avail(tnapi) <= skb_shinfo(skb)->gso_segs)) {
> > + trace_printk("stopping queue, %d <= %d\n",
> > + tg3_tx_avail(tnapi), skb_shinfo(skb)->gso_segs);
> > netif_tx_stop_queue(txq);
> > + trace_printk("stopped queue\n");
> > + tnapi->wakeup_thresh = skb_shinfo(skb)->gso_segs;
> > + BUG_ON(tnapi->wakeup_thresh >= tnapi->tx_pending);
> >
> > /* netif_tx_stop_queue() must be done before checking
> > * checking tx index in tg3_tx_avail() below, because in
>
> I don't quite understand this logic and I must be missing something.
> gso_segs is the number of TCP segments the large packet will be broken
> up into. If it exceeds dev->gso_max_segs, it means it exceeds
> hardware's capabilty and it will do GSO instead of TSO. But in this
> case in tg3_tso_bug(), we are doing GSO and we may not have enough DMA
> descriptors to do GSO. Each gso_seg typically requires 2 DMA
> descriptors.

You're right, I had wrongly assumed that the skbs coming out of
skb_gso_segment() were linear. I'll address that in v2 of the patch by masking
out NETIF_F_SG in tg3_tso_bug().

I noticed another issue that had not occurred to me: when tg3_tso_bug is
submitting a full gso segs sequence to tg3_start_xmit, the code at the end of
that function stops the queue before the end of the sequence because tx_avail
becomes smaller than (MAX_SKB_FRAGS + 1). The transmission actually proceeds
because tg3_tso_bug() does not honour the queue state but it seems rather
unsightly to me. I'm trying different solutions to this and will resubmit.


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