linux-kernel - Re: [PATCH 2/3] netpoll: rework skb transmit queue

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Message-Id: <20061020.134209.85688168.davem@davemloft.net>
Date:	Fri, 20 Oct 2006 13:42:09 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...l.org
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] netpoll: rework skb transmit queue

From: Stephen Hemminger <shemminger@...l.org>
Date: Fri, 20 Oct 2006 08:40:15 -0700

> -static void queue_process(void *p)
> +static void netpoll_run(unsigned long arg)
>  {
 ...
> -		spin_unlock_irqrestore(&queue_lock, flags);
> +		netif_tx_lock(dev);
> +		if (netif_queue_stopped(dev) ||
> +		    dev->hard_start_xmit(skb, dev) != NETDEV_TX_OK) {
> +			skb_queue_head(&npinfo->tx_q, skb);
> +			netif_tx_unlock(dev);
> +			tasklet_schedule(&npinfo->tx_task);
> +			return;
> +		}

We really can't handle TX stopped this way from the netpoll_send_skb()
path.  All that old retry logic in netpoll_send_skb() is really
necessary.

If we are in deep IRQ context, took an OOPS, and are trying to get a
netpoll packet out for the kernel log message, we have to try as hard
as possible to get the packet out then and there, even if that means
waiting some amount of time for netif_queue_stopped() to become false.

That is what the existing code is trying to do.

If you defer to a tasklet, the kernel state from the OOPS can be so
corrupted that the tasklet will never run and we'll never get the
netconsole message needed to debug the problem.

Also, if we tasklet schedule from the tasklet, we'll just keep looping
in the tasklet and never leave softirq context, which is also bad
behavior.  Even in the tasklet, we should spin and poll when possible
like the current netpoll_send_skb() code does.

So we really can't apply this patch.
-
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