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Subject[PATCH] virtio_net: invoke softirqs after __napi_schedule
__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@redhat.com>
Tested-by: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
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?

drivers/net/virtio_net.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index af8acc8..cbefe67 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -492,7 +492,9 @@ static void virtnet_napi_enable(struct virtnet_info *vi)
* We synchronize against interrupts via NAPI_STATE_SCHED */
if (napi_schedule_prep(&vi->napi)) {
virtqueue_disable_cb(vi->rvq);
+ local_bh_disable();
__napi_schedule(&vi->napi);
+ local_bh_enable();
}
}

--
MST

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