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

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Date: Wed, 5 Jul 2023 11:11:19 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Wei Fang <wei.fang@....com>
Cc: Andrew Lunn <andrew@...n.ch>, "davem@...emloft.net"
 <davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
 "pabeni@...hat.com" <pabeni@...hat.com>, "ast@...nel.org" <ast@...nel.org>,
 "daniel@...earbox.net" <daniel@...earbox.net>, "hawk@...nel.org"
 <hawk@...nel.org>, "john.fastabend@...il.com" <john.fastabend@...il.com>,
 Shenwei Wang <shenwei.wang@....com>, Clark Wang <xiaoning.wang@....com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, dl-linux-imx
 <linux-imx@....com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH net 3/3] net: fec: increase the size of tx ring and
 update thresholds of tx ring

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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