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:

quoted

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 <redacted>

Acked-by: Michael S. Tsirkin <mst@redhat.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 <redacted>

quoted

---
  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