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From: Stefan Hajnoczi <stefanha@gmail.com>
Date: Mon, 27 Jun 2016 16:09:48 +0100
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Subject: Re: [Qemu-devel] [PATCH 1/1] Revert "linux-aio: Cancel BH if not
 needed"
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Roman Pen <roman.penyaev@profitbricks.com>, Paolo Bonzini <pbonzini@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

On Fri, Jun 24, 2016 at 3:46 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 24.06.2016 um 15:40 hat Roman Pen geschrieben:
>> This reverts commit ccb9dc10129954d0bcd7814298ed445e684d5a2a,
>> which causes MQ stuck while doing IO thru virtio_blk.
>
> It would be good to have a theory why this happens.

It's worth taking the batch notify BH out of the equation in
virtio_blk_data_plane_notify():

-    set_bit(virtio_get_queue_index(vq), s->batch_notify_vqs);
-    qemu_bh_schedule(s->bh);
+    if (virtio_should_notify(s->vdev, vq)) {
+        event_notifier_set(virtio_queue_get_guest_notifier(vq));
+    }

I wonder if that makes any difference?

I don't have a concrete theory why batch notify interferes with
Kevin's patch though.

Stefan