Subject: Re: [PATCH] get 2% or more performance improved by reducing spin_lock race

From: Sergei Shtylyov <[email-masked]>
Date: Fri, 07 Jun 2013 16:36:37 +0400

Hello.

On 07-06-2013 11:31, Qinchuanyu wrote:

> the wake_up_process func is included by spin_lock/unlock in vhost_work_queue,
>   but it could be done outside the spin_lock.
> I have test it with kernel 3.0.27 and guest suse11-sp2 using iperf, the 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

    Could you align your columns?

> Signed-off-by: Chuanyu Qin <[email-masked]>
> ---
>   drivers/vhost/vhost.c |    5 +++--
>   1 files changed, 3 insertions(+), 2 deletions(-)

> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 94dbd25..8bee109 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -146,9 +146,10 @@ static inline void vhost_work_queue(struct vhost_dev *dev,
>   	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);

    You should have {} in the *else* branch, if you have it in the *if* 
branch (and vice versa), according to Documentation/CodingStyle.

WBR, Sergei