netdev - Re: Regression in throughput between kvm guests over virtual bridge

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Message-Id: <038445a6-9dd5-30c2-aac0-ab5efbfa7024@linux.vnet.ibm.com>
Date:   Thu, 5 Oct 2017 16:07:45 -0400
From:   Matthew Rosato <mjrosato@...ux.vnet.ibm.com>
To:     Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, mst@...hat.com
Subject: Re: Regression in throughput between kvm guests over virtual bridge

On 09/25/2017 04:18 PM, Matthew Rosato wrote:
> On 09/22/2017 12:03 AM, Jason Wang wrote:
>>
>>
>> On 2017年09月21日 03:38, Matthew Rosato wrote:
>>>> Seems to make some progress on wakeup mitigation. Previous patch tries
>>>> to reduce the unnecessary traversal of waitqueue during rx. Attached
>>>> patch goes even further which disables rx polling during processing tx.
>>>> Please try it to see if it has any difference.
>>> Unfortunately, this patch doesn't seem to have made a difference.  I
>>> tried runs with both this patch and the previous patch applied, as well
>>> as only this patch applied for comparison (numbers from vhost thread of
>>> sending VM):
>>>
>>> 4.12    4.13     patch1   patch2   patch1+2
>>> 2.00%   +3.69%   +2.55%   +2.81%   +2.69%   [...] __wake_up_sync_key
>>>
>>> In each case, the regression in throughput was still present.
>>
>> This probably means some other cases of the wakeups were missed. Could
>> you please record the callers of __wake_up_sync_key()?
>>
> 
> Hi Jason,
> 
> With your 2 previous patches applied, every call to __wake_up_sync_key
> (for both sender and server vhost threads) shows the following stack trace:
> 
>      vhost-11478-11520 [002] ....   312.927229: __wake_up_sync_key
> <-sock_def_readable
>      vhost-11478-11520 [002] ....   312.927230: <stack trace>
>  => dev_hard_start_xmit
>  => sch_direct_xmit
>  => __dev_queue_xmit
>  => br_dev_queue_push_xmit
>  => br_forward_finish
>  => __br_forward
>  => br_handle_frame_finish
>  => br_handle_frame
>  => __netif_receive_skb_core
>  => netif_receive_skb_internal
>  => tun_get_user
>  => tun_sendmsg
>  => handle_tx
>  => vhost_worker
>  => kthread
>  => kernel_thread_starter
>  => kernel_thread_starter
> 

Ping...  Jason, any other ideas or suggestions?