Subject: Re: [PATCH] vhost: get 2% performance improved by reducing spin_lock race in vhost_work_queue

From: Jason Wang <[email-masked]>
Date: Mon, 20 May 2013 12:46:55 +0800

On 05/20/2013 12:22 PM, Qinchuanyu wrote:
> The patch below is base on 
> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/drivers/vhost/vhost.c?id=refs/tags/next-20130517
>
> Signed-off-by: Chuanyu Qin <[email-masked]>
> --- a/drivers/vhost/vhost.c     2013-05-20 11:47:05.000000000 +0800
> +++ b/drivers/vhost/vhost.c     2013-05-20 11:48:24.000000000 +0800
> @@ -154,9 +154,10 @@
>         if (list_empty(&work->node)) {
>                 list_add_tail(&work->node, &dev->work_list);
>                 work->queue_seq++;
> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>                 wake_up_process(dev->worker);
> -       }
> -       spin_unlock_irqrestore(&dev->work_lock, flags);
> +       } else
> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>  }
>  
>  void vhost_poll_queue(struct vhost_poll *poll)
>
> I did the test by using iperf in 10G environment, the test num as below:
>                  orignal                   modified
> thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
> 1           9.59         28.82   |      9.59        27.49
> 8            9.61        32.92   |      9.62        26.77
> 64            9.58        46.48  |     9.55        38.99
> 256            9.6        63.7   |      9.6         52.59
>
> The cost of vhost reduced while the throughput is almost unchanged.

Thanks, and please generate a formal patch based on
Documentation/SubmittingPatches (put the description and perf numbers in
the commit log). Then resubmit it to let the maintainer apply it.

>
> On 05/20/2013 11:06 AM, Qinchuanyu wrote:
>> Right now the wake_up_process func is included in spin_lock/unlock, but it could be done outside the spin_lock.
>> I have test it with kernel 3.0.27 and guest suse11-sp2, it provide 2%-3% net performance improved.
>>
>> Signed-off-by: Chuanyu Qin <[email-masked]>
> Make sense to me but need generate a patch against net-next.git or
> vhost.git in git.kernel.org.
>
> Btw. How did you test this? Care to share the perf numbers?
>
> Thanks
>> mu
>> --- a/drivers/vhost/vhost.c     2013-05-20 10:36:30.000000000 +0800
>> +++ b/drivers/vhost/vhost.c     2013-05-20 10:36:54.000000000 +0800
>> @@ -144,9 +144,10 @@
>>         if (list_empty(&work->node)) {
>>                 list_add_tail(&work->node, &dev->work_list);
>>                 work->queue_seq++;
>> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>>                 wake_up_process(dev->worker);
>> -       }
>> -       spin_unlock_irqrestore(&dev->work_lock, flags);
>> +       } else
>> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>>  }
>>  
>>  void vhost_poll_queue(struct vhost_poll *poll)
>> --
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