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 20:11:55 -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 Thu, 6 Jul 2023 01:44:49 +0000 Wei Fang wrote:
> > 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.
> >   
> Maybe I should keep the tx_stop_threshold unchanged, so that the queue is
> to be stopped if the available BDs is not enough for a full TSO frame to be attached.
> And then just change tx_wake_threshold to tx_stop_threshold + 1, which I think it's
> more reasonable.

How about at least tx_stop_threshold + 2 * MAX_SKB_FRAGS ?
If a queue of hundreds of entries is overflowing, we should be able 
to apply a hysteresis of a few tens of entries. Do you see a
difference in drops? The packets from the stack should preferably stay
in the qdiscs instead of the driver queue, where AQM and scheduling can
be applied.