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Date: Mon, 16 Apr 2012 16:39:00 +0300 From: "Michael S. Tsirkin" <mst@...hat.com> To: Jason Wang <jasowang@...hat.com> Cc: netdev@...r.kernel.org, xma@...ibm.com, davem@...emloft.net, linux-kernel@...r.kernel.org, ebiederm@...ssion.com Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote: > On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote: > >On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote: > >>Currently, we restart tx polling unconditionally when sendmsg() > >>fails. This would cause unnecessary wakeups of vhost wokers as it's > >>only needed when the socket send buffer were exceeded. > >Why is this a problem? > > This issue is when guest driver is able to hit the -EFAULT, vhost > discard the the descriptor and restart the polling. This would wake > vhost thread and repeat the loop again which waste cpu. Does same thing happen if we get an error from copy from user? > Another possible solution is don't discard the descriptor. > > > >>Fix this by > >>restart the tx polling only when sendmsg() returns value other than > >>-EFAULT. > >> > >>Signed-off-by: Jason Wang<jasowang@...hat.com> > >>--- > >> drivers/vhost/net.c | 3 ++- > >> 1 files changed, 2 insertions(+), 1 deletions(-) > >> > >>diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > >>index 29abd65..035fa95 100644 > >>--- a/drivers/vhost/net.c > >>+++ b/drivers/vhost/net.c > >>@@ -262,7 +262,8 @@ static void handle_tx(struct vhost_net *net) > >> UIO_MAXIOV; > >> } > >> vhost_discard_vq_desc(vq, 1); > >>- tx_poll_start(net, sock); > >>+ if (err != -EFAULT) > >>+ tx_poll_start(net, sock); > >> break; > >> } > >> if (err != len) > >-- > >To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > >the body of a message to majordomo@...r.kernel.org > >More majordomo info at http://vger.kernel.org/majordomo-info.html > >Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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