Re: [PATCH net-next RFC 2/2] vhost_net: basic polling support
flat view
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2015-10-22 09:33:52
Also in:
kvm, lkml, netdev
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 <redacted>
We might be able to shave off the minor regression by careful use of likely/unlikely, or maybe deferring
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--- 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.
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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?
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+ + return busyloop_timeout && !need_resched() && + !time_after(now, endtime) && !vhost_has_work(dev) && + single_task_running();
signal pending as well?
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+} + /* 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