linux-kernel - Re: [PATCH] vhost-net: add time limitation for tx polling

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Date:   Tue, 27 Mar 2018 17:40:28 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     haibinzhang(张海斌) 
        <haibinzhang@...cent.com>, "mst@...hat.com" <mst@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vhost-net: add time limitation for tx polling



On 2018年03月27日 17:12, haibinzhang(张海斌) wrote:
> handle_tx() will delay rx for a long time when busy tx polling udp packets
> with short length(ie: 1byte udp payload), because setting VHOST_NET_WEIGHT
> takes into account only sent-bytes but no time.

Interesting.

Looking at vhost_can_busy_poll() it tries to poke pending vhost work and 
exit the busy loop if it found one. So I believe something block the 
work queuing. E.g did reverting 8241a1e466cd56e6c10472cac9c1ad4e54bc65db 
fix the issue?

>   It's not fair for handle_rx(),
> so needs to limit max time of tx polling.
>
> Signed-off-by: Haibin Zhang <haibinzhang@...cent.com>
> ---
>   drivers/vhost/net.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8139bc70ad7d..dc9218a3a75b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -473,6 +473,7 @@ static void handle_tx(struct vhost_net *net)
>   	struct socket *sock;
>   	struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
>   	bool zcopy, zcopy_used;
> +	unsigned long start = jiffies;

Checking jiffies is tricky, need to convert it to ms or whatever others.

>   
>   	mutex_lock(&vq->mutex);
>   	sock = vq->private_data;
> @@ -580,7 +581,7 @@ static void handle_tx(struct vhost_net *net)
>   		else
>   			vhost_zerocopy_signal_used(net, vq);
>   		vhost_net_tx_packet(net);
> -		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
> +		if (unlikely(total_len >= VHOST_NET_WEIGHT) || unlikely(jiffies - start >= 1)) {

How value 1 is determined here? And we need a complete test to make sure 
this won't affect other use cases.

Another thought is introduce another limit of #packets, but this need 
benchmark too.

Thanks

>   			vhost_poll_queue(&vq->poll);
>   			break;
>   		}