netdev - Re: pktgen and spin_lock

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Message-ID: <OFE19FAAE4.E89690B3-ON65257656.001B6B90-65257656.001CA0C0@in.ibm.com>
Date:	Wed, 21 Oct 2009 10:42:41 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	NetDev <netdev@...r.kernel.org>, netdev-owner@...r.kernel.org,
	robert@...julf.net
Subject: Re: pktgen and spin_lock_bh in xmit path

Ben Greear <greearb@...delatech.com>  wrote on 10/21/2009 02:40:13 AM:

Coming back a bit to this post:

> > -   queue_map = skb_get_queue_mapping(pkt_dev->skb);
> > +   queue_map = pkt_dev->cur_queue_map;
> > +   /*
> > +    * tells skb_tx_hash() to use this tx queue.
> > +    * We should reset skb->mapping before each xmit() because
> > +    * xmit() might change it.
> > +    */
> > +   skb_record_rx_queue(pkt_dev->skb, queue_map);
> >      txq = netdev_get_tx_queue(odev, queue_map);
>
> I think that must be wrong.  The record_rx_queue sets it to queue_map +
1,
> but the hard-start-xmit method (in ixgbe/ixgbe_main.c, at least), takes
the
> skb->queue_map and uses it as an index with no subtraction.

But that should work fine. record_rx_q sets queue_mapping to +1,
but skb_tx_hash calls skb_get_rx_queue, which does a -1 on this
value, and updates that value into queue_mapping. Hence it will
not cross the txq boundary. Drivers can use the queue_map value
directly without requiring to subtract.

Thanks,

- KK

>
> This causes watchdog timeouts because we are calling txq_trans_update in
pktgen on
> queue 0, for instance, but sending pkts on queue 1.  If queue 1 is ever
> busy when the WD fires, link is reset.

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