Subject: Re: [PATCH] virtio_net: invoke softirqs after __napi_schedule

From: Rusty Russell <[email-masked]>
Date: Thu, 17 May 2012 13:02:53 +0930

On Wed, 16 May 2012 10:57:13 +0300, "Michael S. Tsirkin" <[email-masked]> 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 <[email-masked]>
> Tested-by: Ulrich Obergfell <[email-masked]>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
> 
> To test, one can hack try_fill_recv to always report oom.
> I'm not sure it's not too late for 3.4, but we can try.
> Rusty, could you review ASAP pls?

It's missing a big comment: it's a very complicated way of calling
do_softirq().

Indeed, this function is only used when we are not in interrupt
context.  It's not hot at all, in any ideal scenario.

Acked-by: Rusty Russell <[email-masked]>