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On Wed, Feb 26, 2014 at 05:20:09PM +0800, Qin Chuanyu wrote:
> guest kick host base on avail_ring flags value and get perfermance
typo
> improved, vhost_zerocopy_callback could do the same thing. As
> virtqueue_enable_cb need one more check after modifying the value of
> avail_ring flags, vhost also need do the same thing after
> vhost_enable_notify.
>
> test result list as below:
> guest and host: suse11sp3, netperf, intel CPU 2.4GHz
> +------+----------+--------+----------+--------+--------+---------+
> | | old | new |
> +------+----------+--------+----------+--------+--------+---------+
> | UDP | Gbit/s | PPS |CPU idle% | Gbit/s | PPS |CPU idle%|
> | 256 | 0.74805 | 321309 | 87.16 | 0.77933| 334743 | 90.71 |
> | 512 | 1.42 | 328475 | 87.03 | 1.44 | 333550 | 90.43 |
> | 1024 | 2.79 | 334426 | 89.09 | 2.81 | 336986 | 89.55 |
> | 1460 | 3.71 | 316215 | 87.53 | 4.02 | 342325 | 89.58 |
> +------+----------+--------+----------+--------+--------+---------+
>
> Signed-off-by: Chuanyu Qin <qinchuanyu@xxxxxxxxxx>
It's an interesting optimization, thanks!
However, it looks like this might delay
updating used ring indefinitely if we are
unlucky. Some guests (e.g. windows)
tend to crash if this happens.
Maybe use a new flag for this?
It also looks like there are potential race conditions below.
> ---
> drivers/vhost/net.c | 13 ++++++++++++-
> 1 files changed, 12 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index a0fa5de..a90f51b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -315,6 +315,10 @@ static void vhost_zerocopy_callback(struct
> ubuf_info *ubuf, bool success)
> VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
> cnt = vhost_net_ubuf_put(ubufs);
>
> + /* make sure len has been updated because handle_tx would use it
> + * and used_flags should also been checked.
> + */
> + smp_mb();
> /*
> * Trigger polling thread if guest stopped submitting new buffers:
> * in this case, the refcount after decrement will eventually reach 1.
this barrier is very suspect.
> @@ -322,7 +326,8 @@ static void vhost_zerocopy_callback(struct
> ubuf_info *ubuf, bool success)
> * (the value 16 here is more or less arbitrary, it's tuned to trigger
> * less than 10% of times).
> */
> - if (cnt <= 1 || !(cnt % 16))
> + if ((!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
> + && (cnt <= 1 || !(cnt % 16)))
> vhost_poll_queue(&vq->poll);
>
> rcu_read_unlock_bh();
looks like a potential race to me
> @@ -386,6 +391,12 @@ static void handle_tx(struct vhost_net *net)
> vhost_disable_notify(&net->dev, vq);
> continue;
> }
> + /* there might skb been freed between last
> + * vhost_zerocopy_signal_used and vhost_enable_notify,
> + * so one more check is needed.
> + */
> + if (zcopy)
> + vhost_zerocopy_signal_used(net, vq);
> break;
> }
> if (in) {
> --
> 1.7.3.1.msysgit.0
>
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