Re: [PATCH V3 2/2] vhost_net: conditionally enable tx polling

Jason Wang
2016-06-08T06:48:18Z

On 2016年06月07日 20:26, Michael S. Tsirkin wrote:
> On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:
>> We always poll tx for socket, this is sub optimal since:
>>
>> - it will be only used when we exceed the sndbuf of the socket.
>> - since we use two independent polls for tx and vq, this will slightly
>> increase the waitqueue traversing time and more important, vhost
>> could not benefit from commit
>> 9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
>> tun_do_read for better sleep/wakeup efficiency") even if we've
>> stopped rx polling during handle_rx since tx poll were still left in
>> the waitqueue.
>>
>> Fix this by conditionally enable tx polling only when -EAGAIN were
>> met.
>>
>> Test shows about 8% improvement on guest rx pps.
>>
>> Before: ~1350000
>> After: ~1460000
>>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>> drivers/vhost/net.c | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 1d3e45f..e75ffcc 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
>> goto out;
>>
>> vhost_disable_notify(&net->dev, vq);
>> + vhost_net_disable_vq(net, vq);
>>
>> hdr_size = nvq->vhost_hlen;
>> zcopy = nvq->ubufs;
>> @@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
>> % UIO_MAXIOV;
>> }
>> vhost_discard_vq_desc(vq, 1);
>> + if (err == -EAGAIN)
>> + vhost_net_enable_vq(net, vq);
>> break;
>> }
>> if (err != len)
> This seems rather risky. What if TX failed for some other reason?
> Polling won't ever be re-enabled ...
>

But why we need to enable tx poll in this case? Even if we enable it, we
wont' get any wakeup.

>> --
>> 1.8.3.1

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