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

[Wei Fang]

> -----Original Message-----
> From: Jakub Kicinski <kuba@xxxxxxxxxx>
> Sent: 2023年7月6日 11:12
> To: Wei Fang <wei.fang@xxxxxxx>
> Cc: Andrew Lunn <andrew@xxxxxxx>; davem@xxxxxxxxxxxxx;
> edumazet@xxxxxxxxxx; pabeni@xxxxxxxxxx; ast@xxxxxxxxxx;
> daniel@xxxxxxxxxxxxx; hawk@xxxxxxxxxx; john.fastabend@xxxxxxxxx;
> Shenwei Wang <shenwei.wang@xxxxxxx>; Clark Wang
> <xiaoning.wang@xxxxxxx>; netdev@xxxxxxxxxxxxxxx; dl-linux-imx
> <linux-imx@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; bpf@xxxxxxxxxxxxxxx
> 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 ?
It's okay. The iperf performance is well as before.

> 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
I didn't see there was any packet loss.

> packets from the stack should preferably stay in the qdiscs instead of the driver
> queue, where AQM and scheduling can be applied.