Subject: Re: [PATCH] vhost: poll vhost_net only when tx notification is enabled

From: Jason Wang <[email-masked]>
Date: Fri, 28 Feb 2014 15:49:22 +0800

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 <[email-masked]>
> 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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