linux-kernel - Re: [PATCH net-next v2 4/4] net: mvneta: Spread out the TX queues management on all CPUs

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Message-Id: <20151205.172437.1660871557891874317.davem@davemloft.net>
Date:	Sat, 05 Dec 2015 17:24:37 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mw@...ihalf.com
Cc:	gregory.clement@...e-electrons.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, thomas.petazzoni@...e-electrons.com,
	jason@...edaemon.net, andrew@...n.ch,
	sebastian.hesselbarth@...il.com,
	ezequiel.garcia@...e-electrons.com,
	linux-arm-kernel@...ts.infradead.org, alior@...vell.com,
	nadavh@...vell.com, simon.guinot@...uanux.org,
	maxime.ripard@...e-electrons.com,
	boris.brezillon@...e-electrons.com, linux@....linux.org.uk,
	w@....eu, arnd@...db.de
Subject: Re: [PATCH net-next v2 4/4] net: mvneta: Spread out the TX queues
 management on all CPUs

From: Marcin Wojtas <mw@...ihalf.com>
Date: Sat, 5 Dec 2015 20:14:31 +0100

> Hi Gregory,
> 
>> @@ -1824,13 +1835,16 @@ error:
>>  static int mvneta_tx(struct sk_buff *skb, struct net_device *dev)
>>  {
>>         struct mvneta_port *pp = netdev_priv(dev);
>> -       u16 txq_id = skb_get_queue_mapping(skb);
>> +       u16 txq_id = smp_processor_id() % txq_number;
> 
> I think it may be ok to bind TXQs to different CPUs, but I don't think
> that replacing skb_get_queue_mapping by in fact smp_processor_id() is
> the best idea. This way you use only 2 TXQs on A385 and 4 TXQs on AXP.
> There are HW mechanisms like WRR or EJP that provide balancing for
> egress, so let's better keep all 8.

Also it is possible for other parts of the stack to set the SKB queue
mapping and you must respect that setting rather than override it.
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