Re: [PATCH] vhost: make vhost_zerocopy_callback more efficient by poll_queue base on vhost status

[Jason Wang <jasowang@redhat.com>]

On 02/25/2014 03:53 PM, Qin Chuanyu wrote:
> On 2014/2/25 15:38, Jason Wang wrote:
>> On 02/25/2014 02:55 PM, Qin Chuanyu wrote:
>>> guest kick vhost base on vring flag status and get perfermance
>>> improved,
>>> vhost_zerocopy_callback could do this in the same way, as
>>> virtqueue_enable_cb need one more check after change the status of
>>> avail_ring flags, vhost also do the same thing after
>>> vhost_enable_notify
>>>
>>> test result list as below:
>>> guest and host: suse11sp3, netperf, intel 2.4GHz
>>> +-------+----------+---------+----------+---------+
>>> |       |   old              |   new              |
>>> +-------+----------+---------+----------+---------+
>>> |  UDP  |  Gbit/s  |   PPS   |  Gbit/s  |   PPS   |
>>> |  256  | 0.74805  | 321309  | 0.77933  | 334743  |
>>> |  512  |   1.42   | 328475  |   1.44   | 333550  |
>>> | 1024  |   2.79   | 334426  |   2.81   | 336986  |
>>> | 1460  |   3.71   | 316215  |   4.02   | 342325  |
>>> +-------+----------+---------+----------+---------+
>>
>> Looks good, do you have cpu utilization number?
> +------+----------+--------+----------+--------+--------+---------+
> |      |             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  |
> +------+----------+--------+----------+--------+--------+---------+
> after change, less cpu has been used.

Thanks
>>>
>>> Signed-off-by: Chuanyu Qin <qinchuanyu@huawei.com>
>>> ---
>>>   drivers/vhost/net.c |   10 +++++++++-
>>>   1 files changed, 9 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>> index a0fa5de..9bc0a15 100644
>>> --- a/drivers/vhost/net.c
>>> +++ b/drivers/vhost/net.c
>>> @@ -322,7 +322,9 @@ 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))
>>> +    smp_rmb();
>>
>> Better add a comment to explain why this is needed.
>>
>> Looks like what you need is a smp_mb() here to make sure the len is
>> updated before testing vq->used_flags?
> I wanner make sure the used_flags is updated, is smp_rmb() enough?
> or a smp_mb() is needed?

used_flags was guaranteed to be updated after smp_mb() in
vhost_enable_notify(). And vhost_net_ubuf_put() does a
atomic_sub_return() which implements memory barrier. So looks like
there's no need for an extra one.

>>> +    if ((!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>>> +            && (cnt <= 1 || !(cnt % 16)))
>>>           vhost_poll_queue(&vq->poll);
>>>
>>>       rcu_read_unlock_bh();
>>> @@ -386,6 +388,12 @@ 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) {
>>
>>
>> .
>>
>
>
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