netdev - Re: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso

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Message-ID: <20140821215911.GA7117@f1.synalogic.ca>
Date:	Thu, 21 Aug 2014 14:59:11 -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 2014/08/21 02:51, Michael Chan wrote:
> On Wed, 2014-08-20 at 18:23 -0700, Benjamin Poirier wrote: 
> > 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().
> > 
> 
> While masking out NETF_F_SG will work, it will also disable checksum
> offload for the whole device momentarily.
> 
> > 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.
> 
> That's why the number of DMA descriptors that we estimate has to be
> accurate.  It's unfortunate that the various tg3 chips require so many
> different workarounds.  The objective is to keep TSO and checksum
> enabled and workaround the occasional packets using GSO.

Ah, now I understand the reason for the * 3 in
	u32 frag_cnt_est = skb_shinfo(skb)->gso_segs * 3;

	/* Estimate the number of fragments in the worst case */
but that is not really the "worst case". It's not forbidden to have more than
two frags per skb output from skb_gso_segment(). I've kept this estimation
approach but I've added code to validate the estimation or else linearize the
skb.

> 
> I believe that the boundary error conditions that you brought up can be
> addressed by enforcing some limits on the tx ring size and by reducing
> gso_max_size/gso_max_segs when necessary (for example when MTU and/or
> ring size is set very small).
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