netdev - Re: [PATCH net-next] virtio_net: Disable interrupts if napi_complete

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Date:   Thu, 7 Dec 2017 15:08:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: Disable interrupts if
 napi_complete_done rescheduled napi



On 2017年12月07日 13:09, Michael S. Tsirkin wrote:
> 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 <makita.toshiaki@....ntt.co.jp>
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
>
> 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?

No sure, the regression I saw was tested with event idx on. And 
virtqueue_disable_cb() does almost nothing for event idx (or even a 
little bit slower).

The patch looks good.

Acked-by: Jason Wang <jasowang@...hat.com>


>
>> ---
>>   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
>>