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Date: Wed, 20 Aug 2014 18:23:03 -0700 From: Benjamin Poirier <bpoirier@...e.de> To: Michael Chan <mchan@...adcom.com> Cc: Prashant Sreedharan <prashant@...adcom.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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