netdev - Re: [BUG] net: cpu offline cause napi stall

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Date:	Wed, 01 Jun 2011 18:55:21 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Frank Blaschka <blaschka@...ux.vnet.ibm.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [BUG] net: cpu offline cause napi stall

Le mercredi 01 juin 2011 à 18:36 +0200, Heiko Carstens a écrit :
> On Wed, Jun 01, 2011 at 02:13:19PM +0200, Eric Dumazet wrote:
> > Le mercredi 01 juin 2011 à 12:33 +0200, Frank Blaschka a écrit :
> > > Hi Dave, Eric,
> > > 
> > > during heavy network load we turn off/on cpus.
> > > Sometimes this causes a stall on the network device.
> > > Digging into the dump I found out following:
> > > 
> > > napi is scheduled but does not run. From the I/O buffers
> > > and the napi state I see napi/rx_softirq processing has stopped
> > > because the budget was reached. napi stays in the
> > > softnet_data poll_list and the rx_softirq was raised again.
> > > 
> > > I assume at this time the cpu offline comes in.
> > > the rx softirq is raised/moved to another cpu but napi stays in the poll_list
> > > of the softnet_data of the now offline cpu.
> > > 
> > > reviewing dev_cpu_callback (net/core/dev.c) I did not find the poll_list
> > > is transfered to the new cpu. Do you think this could cause the stall or
> > > did I miss something?
> > > 
> > > Thx for your help.
> > 
> > Hi Frank
> > 
> > I believe you are right, I cant see where the poll_list transfert from
> > dead cpu to online cpu is done.
> > 
> > Do you want to prepare a patch ?
> 
> Frank will be offline until next week. I assume the patch below would fix
> the problem, however its untested. I doubt we can verify that it really
> fixes the problem also until next week (public holiday tomorrow).
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 6561021..6d6a7cf 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5981,6 +5981,8 @@ static int dev_cpu_callback(struct notifier_block *nfb,
>  		oldsd->output_queue = NULL;
>  		oldsd->output_queue_tailp = &oldsd->output_queue;
>  	}
> +	/* Append NAPI poll list from offline CPU. */
> +	list_splice_init(&oldsd->poll_list, &sd->poll_list);
>  
>  	raise_softirq_irqoff(NET_TX_SOFTIRQ);
>  	local_irq_enable();

Same here, I'll be offline for the next 4 days ;)

Please make sure we raise NET_RX_SOFTIRQ on new cpu if necessary.



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