netdev - Re: [PATCH] virtio_net: invoke softirqs after __napi

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Date:	Wed, 16 May 2012 23:40:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	mst@...hat.com, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_net: invoke softirqs after __napi_schedule

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Thu, 17 May 2012 13:02:53 +0930

> On Wed, 16 May 2012 10:57:13 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
>> __napi_schedule might raise softirq but nothing
>> causes do_softirq to trigger, so it does not in fact
>> run. As a result,
>> the error message "NOHZ: local_softirq_pending 08"
>> sometimes occurs during boot of a KVM guest when the network service is
>> started and we are oom:
>> 
>>   ...
>>   Bringing up loopback interface:  [  OK  ]
>>   Bringing up interface eth0:
>>   Determining IP information for eth0...NOHZ: local_softirq_pending 08
>>    done.
>>   [  OK  ]
>>   ...
>> 
>> Further, receive queue processing might get delayed
>> indefinitely until some interrupt triggers:
>> virtio_net expected napi to be run immediately.
>> 
>> One way to cause do_softirq to be executed is by
>> invoking local_bh_enable(). As __napi_schedule is
>> normally called from bh or irq context, this
>> seems to make sense: disable bh before __napi_schedule
>> and enable afterwards.
>> 
>> Reported-by: Ulrich Obergfell <uobergfe@...hat.com>
>> Tested-by: Ulrich Obergfell <uobergfe@...hat.com>
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
 ...
> Acked-by: Rusty Russell <rusty@...tcorp.com.au>

Michael, you're best to submit this directly to Linus as I just
made what I hope is my last push to him for 3.4 today.
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