From: Rusty Russell <rusty@rustcorp.com.au>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: pagupta@redhat.com, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org
Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending
Date: Wed, 11 Feb 2015 10:28:38 +1030 [thread overview]
Message-ID: <87zj8lb9ox.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20150210101839.GA9505@redhat.com>
"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Feb 10, 2015 at 11:33:52AM +1030, Rusty Russell wrote:
>> Jason Wang <jasowang@redhat.com> writes:
>> > We currently does:
>> >
>> > bufs = (avail->idx - last_used_idx) * 3 / 4;
>> >
>> > This is ok now since we only try to enable the delayed callbacks when
>> > the queue is about to be full. This may not work well when there is
>> > only one pending buffer in the virtqueue (this may be the case after
>> > tx interrupt was enabled). Since virtqueue_enable_cb() will return
>> > false which may cause unnecessary triggering of napis. This patch
>> > correct this by only calculate the four thirds when bufs is not one.
>>
>> I mildly prefer to avoid the branch, by changing the calculation like
>> so:
>>
>> /* Set bufs >= 1, even if there's only one pending buffer */
>> bufs = (bufs + 1) * 3 / 4;
>
> Or bus * 3/4 + 1
>
>> But it's not clear to me how much this happens. I'm happy with the
>> patch though, as currently virtqueue_enable_cb_delayed() is the same
>> as virtqueue_enable_cb() if there's only been one buffer added.
>>
>> Cheers,
>> Rusty.
>
> But isn't this by design?
> The documentation says:
>
> * This re-enables callbacks but hints to the other side to delay
> * interrupts until most of the available buffers have been processed;
>
> Clearly, this implies that when there's one buffer it must behave
> exactly the same.
Yes, my mistake. We could hit the "never gets notified" case with this
change, and that's a much bigger problem.
So I don't think we can accept this patch...
Rusty.
next prev parent reply other threads:[~2015-02-10 23:58 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-09 8:39 [PATCH RFC v5 net-next 0/6] enable tx interrupts for virtio-net Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending Jason Wang
2015-02-10 1:03 ` Rusty Russell
2015-02-10 6:26 ` Jason Wang
2015-02-10 10:18 ` Michael S. Tsirkin
2015-02-10 23:58 ` Rusty Russell [this message]
2015-02-11 5:41 ` Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 2/6] virtio_ring: try to disable event index callbacks in virtqueue_disable_cb() Jason Wang
2015-02-10 1:07 ` Rusty Russell
2015-02-10 10:24 ` Michael S. Tsirkin
2015-02-11 5:55 ` Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 3/6] virtio_net: enable tx interrupt Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 4/6] virtio-net: add basic interrupt coalescing support Jason Wang
2015-02-10 1:32 ` Rusty Russell
2015-02-10 6:51 ` Jason Wang
2015-02-10 10:25 ` Michael S. Tsirkin
2015-02-10 10:40 ` Michael S. Tsirkin
2015-02-13 2:52 ` Rusty Russell
2015-02-13 12:41 ` Pawel Moll
2015-02-16 3:07 ` Rusty Russell
2015-02-13 18:19 ` Cornelia Huck
2015-02-16 3:19 ` Rusty Russell
2015-02-09 8:39 ` [PATCH RFC v5 net-next 5/6] vhost: let vhost_signal() returns whether signalled Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 6/6] vhost_net: interrupt coalescing support Jason Wang
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