linux-kernel - [PATCHv2 net-next 8/8] vhost-net: reduce vq polling on tx zerocopy

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Date:	Wed, 31 Oct 2012 12:31:20 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	Vlad Yasevich <vyasevic@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Ian Campbell <Ian.Campbell@...rix.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCHv2 net-next 8/8] vhost-net: reduce vq polling on tx zerocopy

It seems that to avoid deadlocks it is enough to poll vq before
 we are going to use the last buffer.  This is faster than
c70aa540c7a9f67add11ad3161096fb95233aa2e.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 drivers/vhost/net.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8e9de79..88beedb 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -197,8 +197,18 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, int status)
 {
 	struct vhost_ubuf_ref *ubufs = ubuf->ctx;
 	struct vhost_virtqueue *vq = ubufs->vq;
-
-	vhost_poll_queue(&vq->poll);
+	int cnt = atomic_read(&ubufs->kref.refcount);
+
+	/*
+	 * Trigger polling thread if guest stopped submitting new buffers:
+	 * in this case, the refcount after decrement will eventually reach 1
+	 * so here it is 2.
+	 * We also trigger polling periodically after each 16 packets
+	 * (the value 16 here is more or less arbitrary, it's tuned to trigger
+	 * less than 10% of times).
+	 */
+	if (cnt <= 2 || !(cnt % 16))
+		vhost_poll_queue(&vq->poll);
 	/* set len to mark this desc buffers done DMA */
 	vq->heads[ubuf->desc].len = status ?
 		VHOST_DMA_FAILED_LEN : VHOST_DMA_DONE_LEN;
-- 
MST
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