netdev - Re: [PATCH net 6/7] net: axienet: fix for TX busy handling

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Date:   Wed, 12 Jan 2022 16:45:18 +0000
From:   Robert Hancock <robert.hancock@...ian.com>
To:     "kuba@...nel.org" <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "radhey.shyam.pandey@...inx.com" <radhey.shyam.pandey@...inx.com>
Subject: Re: [PATCH net 6/7] net: axienet: fix for TX busy handling

On Tue, 2022-01-11 at 19:49 -0800, Jakub Kicinski wrote:
> On Tue, 11 Jan 2022 15:13:57 -0600 Robert Hancock wrote:
> > We should be avoiding returning NETDEV_TX_BUSY from ndo_start_xmit in
> > normal cases. Move the main check for a full TX ring to the end of the
> > function so that we stop the queue after the last available space is used
> > up, and only wake up the queue if enough space is available for a full
> > maximally fragmented packet. Print a warning if there is insufficient
> > space at the start of start_xmit, since this should no longer happen.
> > 
> > Fixes: 8a3b7a252dca9 ("drivers/net/ethernet/xilinx: added Xilinx AXI
> > Ethernet driver")
> > Signed-off-by: Robert Hancock <robert.hancock@...ian.com>
> 
> Feels a little more like an optimization than strictly a fix.
> Can we apply this and the following patch to net-next in two
> week's time? It's not too much of a stretch to take it in now
> if it's a bit convenience but I don't think the Fixes tags should 
> stay.

Well it's a fix in the sense that it complies with what
Documentation/networking/driver.rst says drivers should do - I'm not too
familiar with the consequences of not doing that are, I guess mostly
performance from having to requeue the packet?

From that standpoint, I guess the concern with breaking those two patches out
is that the previous patches can introduce a bit of a performance hit (by
actually caring about the state of the TX ring instead of trampling over it in
some cases) and so without the last two you might end up with some performance 
regression. So I'd probably prefer to keep them together with the rest of the
patch set.

> 
> > -		netif_wake_queue(ndev);
> > +		netdev_warn(ndev, "TX ring unexpectedly full\n");
> 
> Probably wise to make this netdev_warn_once() or at least rate limit it.

Might want it more than once (so you can tell if it is a one-off or happening
more often), but I can put in a rate limit..

> 
> > +		return NETDEV_TX_BUSY;
> >  	}
> >  
> >  	if (skb->ip_summed == CHECKSUM_PARTIAL) {

-- 
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com