On 02/26/2014 07:16 PM, Michael S. Tsirkin wrote:
> Please see MAINTAINERS and copy all relevant lists.
>
> On Wed, Feb 26, 2014 at 05:20:09PM +0800, Qin Chuanyu wrote:
>> guest kick host base on avail_ring flags value and get perfermance
> typo
>
>> improved, vhost_zerocopy_callback could do the same thing. As
>> virtqueue_enable_cb need one more check after modifying the value of
>> avail_ring flags, vhost also need do the same thing after
>> vhost_enable_notify.
>>
>> test result list as below:
>> guest and host: suse11sp3, netperf, intel CPU 2.4GHz
>> +------+----------+--------+----------+--------+--------+---------+
>> | | old | new |
>> +------+----------+--------+----------+--------+--------+---------+
>> | UDP | Gbit/s | PPS |CPU idle% | Gbit/s | PPS |CPU idle%|
>> | 256 | 0.74805 | 321309 | 87.16 | 0.77933| 334743 | 90.71 |
>> | 512 | 1.42 | 328475 | 87.03 | 1.44 | 333550 | 90.43 |
>> | 1024 | 2.79 | 334426 | 89.09 | 2.81 | 336986 | 89.55 |
>> | 1460 | 3.71 | 316215 | 87.53 | 4.02 | 342325 | 89.58 |
>> +------+----------+--------+----------+--------+--------+---------+
>>
>> Signed-off-by: Chuanyu Qin <qinchuanyu@xxxxxxxxxx>
> It's an interesting optimization, thanks!
> However, it looks like this might delay
> updating used ring indefinitely if we are
> unlucky. Some guests (e.g. windows)
> tend to crash if this happens.
Looks like it does not change this.
When tx notification is disabled, it means handle_tx() is running which
will update the used ring before each tx descriptor fetching. And the
patch does used updating after each time when tx notification is enabled.
>
> Maybe use a new flag for this?
>
> It also looks like there are potential race conditions below.
>
>> ---
>> drivers/vhost/net.c | 13 ++++++++++++-
>> 1 files changed, 12 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index a0fa5de..a90f51b 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -315,6 +315,10 @@ static void vhost_zerocopy_callback(struct
>> ubuf_info *ubuf, bool success)
>> VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
>> cnt = vhost_net_ubuf_put(ubufs);
>>
>> + /* make sure len has been updated because handle_tx would use it
>> + * and used_flags should also been checked.
>> + */
>> + smp_mb();
>> /*
>> * Trigger polling thread if guest stopped submitting new buffers:
>> * in this case, the refcount after decrement will eventually reach 1.
> this barrier is very suspect.
>
>> @@ -322,7 +326,8 @@ static void vhost_zerocopy_callback(struct
>> ubuf_info *ubuf, bool success)
>> * (the value 16 here is more or less arbitrary, it's tuned to trigger
>> * less than 10% of times).
>> */
>> - if (cnt <= 1 || !(cnt % 16))
>> + if ((!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>> + && (cnt <= 1 || !(cnt % 16)))
>> vhost_poll_queue(&vq->poll);
>>
>> rcu_read_unlock_bh();
> looks like a potential race to me
Could you please explain the race a little bit more?
>> @@ -386,6 +391,12 @@ static void handle_tx(struct vhost_net *net)
>> vhost_disable_notify(&net->dev, vq);
>> continue;
>> }
>> + /* there might skb been freed between last
>> + * vhost_zerocopy_signal_used and vhost_enable_notify,
>> + * so one more check is needed.
>> + */
>> + if (zcopy)
>> + vhost_zerocopy_signal_used(net, vq);
>
>> break;
>> }
>> if (in) {
>> --
>> 1.7.3.1.msysgit.0
>>
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