Subject: Re: [PATCH net-next] virtio_net: Disable interrupts if napi_complete_done rescheduled napi

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 7 Dec 2017 07:09:37 +0200

On Thu, Dec 07, 2017 at 01:15:15PM +0900, Toshiaki Makita wrote:
> Since commit 39e6c8208d7b ("net: solve a NAPI race") napi has been able
> to be rescheduled within napi_complete_done() even in non-busypoll case,
> but virtnet_poll() always enabled interrupts before complete, and when
> napi was rescheduled within napi_complete_done() it did not disable
> interrupts.
> This caused more interrupts when event idx is disabled.
> 
> According to commit cbdadbbf0c79 ("virtio_net: fix race in RX VQ
> processing") we cannot place virtqueue_enable_cb_prepare() after
> NAPI_STATE_SCHED is cleared, so disable interrupts again if
> napi_complete_done() returned false.
> 
> Tested with vhost-user of OVS 2.7 on host, which does not have the event
> idx feature.
> 
> * Before patch:
> 
> $ netperf -t UDP_STREAM -H 192.168.150.253 -l 60 -- -m 1472
> MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.150.253 () port 0 AF_INET
> Socket  Message  Elapsed      Messages
> Size    Size     Time         Okay Errors   Throughput
> bytes   bytes    secs            #      #   10^6bits/sec
> 
> 212992    1472   60.00     32763206      0    6430.32
> 212992           60.00     23384299           4589.56
> 
> Interrupts on guest: 9872369
> Packets/interrupt:   2.37
> 
> * After patch
> 
> $ netperf -t UDP_STREAM -H 192.168.150.253 -l 60 -- -m 1472
> MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.150.253 () port 0 AF_INET
> Socket  Message  Elapsed      Messages
> Size    Size     Time         Okay Errors   Throughput
> bytes   bytes    secs            #      #   10^6bits/sec
> 
> 212992    1472   60.00     32794646      0    6436.49
> 212992           60.00     32793501           6436.27
> 
> Interrupts on guest: 4941299
> Packets/interrupt:   6.64
> 
> Signed-off-by: Toshiaki Makita <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

it might make sense in net and not -next since tx napi regressed performance
in some configs, this might bring it back at least partially.
Jason - what do you think?

> ---
>  drivers/net/virtio_net.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 19a985e..c0db48d 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -261,9 +261,12 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
>  	int opaque;
>  
>  	opaque = virtqueue_enable_cb_prepare(vq);
> -	if (napi_complete_done(napi, processed) &&
> -	    unlikely(virtqueue_poll(vq, opaque)))
> -		virtqueue_napi_schedule(napi, vq);
> +	if (napi_complete_done(napi, processed)) {
> +		if (unlikely(virtqueue_poll(vq, opaque)))
> +			virtqueue_napi_schedule(napi, vq);
> +	} else {
> +		virtqueue_disable_cb(vq);
> +	}
>  }
>  
>  static void skb_xmit_done(struct virtqueue *vq)
> -- 
> 1.8.3.1
>