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

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Date:   Thu, 6 Jul 2023 06:14:54 +0000
From:   Wei Fang <wei.fang@....com>
To:     Jakub Kicinski <kuba@...nel.org>
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

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: 2023年7月6日 11:12
> To: Wei Fang <wei.fang@....com>
> Cc: Andrew Lunn <andrew@...n.ch>; davem@...emloft.net;
> edumazet@...gle.com; pabeni@...hat.com; ast@...nel.org;
> daniel@...earbox.net; hawk@...nel.org; john.fastabend@...il.com;
> Shenwei Wang <shenwei.wang@....com>; Clark Wang
> <xiaoning.wang@....com>; netdev@...r.kernel.org; dl-linux-imx
> <linux-imx@....com>; linux-kernel@...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 ?
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.