Subject: Re: [PATCH net 3/3] net: fec: increase the size of tx ring and update thresholds of tx ring

From: Jakub Kicinski <[email-masked]>
Date: Wed, 5 Jul 2023 11:11:19 -0700

On Wed, 5 Jul 2023 06:20:26 +0000 Wei Fang wrote:
> > > In addtion, this patch also updates the tx_stop_threshold and the
> > > tx_wake_threshold of the tx ring. In previous logic, the value of
> > > tx_stop_threshold is 217, however, the value of tx_wake_threshold is
> > > 147, it does not make sense that tx_wake_threshold is less than
> > > tx_stop_threshold.  
> > 
> > What do these actually mean? I could imagine that as the ring fills you don't
> > want to stop until it is 217/512 full. There is then some hysteresis, such that it
> > has to drop below 147/512 before more can be added?
> >   
> You must have misunderstood, let me explain more clearly, the queue will be
> stopped when the available BDs are less than tx_stop_threshold (217 BDs). And
> the queue will be waked when the available BDs are greater than tx_wake_threshold
> (147 BDs). So in most cases, the available BDs are greater than tx_wake_threshold
> when the queue is stopped, the only effect is to delay packet sending.
> In my opinion, tx_wake_threshold should be greater than tx_stop_threshold, we
> should stop queue when the available BDs are not enough for a skb to be attached.
> And wake the queue when the available BDs are sufficient for a skb.

But you shouldn't restart the queue for a single packet either.
Restarting for a single packet wastes CPU cycles as there will 
be much more stop / start operations. Two large packets seem 
like the absolute minimum reasonable wake threshold.

Setting tx_stop_threshold to MAX_SKB_FRAGS doesn't seem right either,
as you won't be able to accept a full TSO frame.

Please split the change, the netdev_err_once() should be one patch
and then the change to wake thresh a separate one.
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