On Thu, Oct 22, 2015 at 01:27:29AM -0400, Jason Wang wrote:
> This patch tries to poll for new added tx buffer for a while at the
> end of tx processing. The maximum time spent on polling were limited
> through a module parameter. To avoid block rx, the loop will end it
> there's new other works queued on vhost so in fact socket receive
> queue is also be polled.
>
> busyloop_timeout = 50 gives us following improvement on TCP_RR test:
>
> size/session/+thu%/+normalize%
> 1/ 1/ +5%/ -20%
> 1/ 50/ +17%/ +3%
Is there a measureable increase in cpu utilization
with busyloop_timeout = 0?
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
We might be able to shave off the minor regression
by careful use of likely/unlikely, or maybe
deferring
> ---
> drivers/vhost/net.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9eda69e..bbb522a 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -31,7 +31,9 @@
> #include "vhost.h"
>
> static int experimental_zcopytx = 1;
> +static int busyloop_timeout = 50;
> module_param(experimental_zcopytx, int, 0444);
> +module_param(busyloop_timeout, int, 0444);
Pls add a description, including the units and the special
value 0.
> MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
> " 1 -Enable; 0 - Disable");
>
> @@ -287,12 +289,23 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
> rcu_read_unlock_bh();
> }
>
> +static bool tx_can_busy_poll(struct vhost_dev *dev,
> + unsigned long endtime)
> +{
> + unsigned long now = local_clock() >> 10;
local_clock might go backwards if we jump between CPUs.
One way to fix would be to record the CPU id and break
out of loop if that changes.
Also - defer this until we actually know we need it?
> +
> + return busyloop_timeout && !need_resched() &&
> + !time_after(now, endtime) && !vhost_has_work(dev) &&
> + single_task_running();
signal pending as well?
> +}
> +
> /* Expects to be always run from workqueue - which acts as
> * read-size critical section for our kind of RCU. */
> static void handle_tx(struct vhost_net *net)
> {
> struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> struct vhost_virtqueue *vq = &nvq->vq;
> + unsigned long endtime;
> unsigned out, in;
> int head;
> struct msghdr msg = {
> @@ -331,6 +344,8 @@ static void handle_tx(struct vhost_net *net)
> % UIO_MAXIOV == nvq->done_idx))
> break;
>
> + endtime = (local_clock() >> 10) + busyloop_timeout;
> +again:
> head = vhost_get_vq_desc(vq, vq->iov,
> ARRAY_SIZE(vq->iov),
> &out, &in,
> @@ -340,6 +355,10 @@ static void handle_tx(struct vhost_net *net)
> break;
> /* Nothing new? Wait for eventfd to tell us they refilled. */
> if (head == vq->num) {
> + if (tx_can_busy_poll(vq->dev, endtime)) {
> + cpu_relax();
> + goto again;
> + }
> if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> vhost_disable_notify(&net->dev, vq);
> continue;
> --
> 1.8.3.1
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