netdev - Re: [PATCH net-next 8/8] vhost-net: reduce vq polling on tx zerocopy

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Date:	Tue, 30 Oct 2012 17:54:12 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Vlad Yasevich <vyasevic@...hat.com>
Cc:	"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: Re: [PATCH net-next 8/8] vhost-net: reduce vq polling on tx zerocopy

On Tue, Oct 30, 2012 at 11:47:45AM -0400, Vlad Yasevich wrote:
> On 10/29/2012 11:49 AM, Michael S. Tsirkin wrote:
> >It seems that to avoid deadlocks it is enough to poll vq before
> >  we are going to use the last buffer.  This should be faster than
> >c70aa540c7a9f67add11ad3161096fb95233aa2e.
> >
> >Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >---
> >  drivers/vhost/net.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> >diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> >index 8e9de79..3967f82 100644
> >--- a/drivers/vhost/net.c
> >+++ b/drivers/vhost/net.c
> >@@ -197,8 +197,16 @@ 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.
> >+	 */
> >+	if (cnt <= 2 || !(cnt % 16))
> 
> Why 16?  Does it make sense to make it configurable?
> 
> -vlad

Probably not but I'll add a comment explaining why.

> >+		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;
> >
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