RE: [PATCH 1/1] net/hyperv: Add flow control based on hi/low watermark

-----Original Message-----
From: Greg KH [mailto:gregkh@linuxfoundation.org]
Sent: Monday, March 26, 2012 7:10 PM
To: Haiyang Zhang
Cc: davem@davemloft.net; netdev@vger.kernel.org;
devel@linuxdriverproject.org; olaf@aepfle.de; linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] net/hyperv: Add flow control based on hi/low
watermark

On Mon, Mar 26, 2012 at 03:18:24PM -0700, Haiyang Zhang wrote:

quoted

In the existing code, we only stop queue when the ringbuffer is full,
so the current packet has to be dropped or retried from upper layer.

This patch stops the tx queue when available ringbuffer is below the
low watermark. So the ringbuffer still has small amount of space
available for the current packet. This will reduce the overhead of
retries on sending.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
---
 drivers/hv/ring_buffer.c        |   15 +++++++++++++++
 drivers/net/hyperv/hyperv_net.h |    3 +++
 drivers/net/hyperv/netvsc.c     |   23 +++++++++++++++++++----
 drivers/net/hyperv/netvsc_drv.c |   16 +++++++++++++++-
 include/linux/hyperv.h          |    3 +++
 5 files changed, 55 insertions(+), 5 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c index

8af25a0..8cc3f63 100644

--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c

@@ -23,6 +23,7 @@
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

+#include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/hyperv.h>

@@ -160,6 +161,20 @@ hv_get_ring_buffersize(struct

hv_ring_buffer_info

quoted

*ring_info)  }

 /*
+ * Get the percentage of available bytes to write in the ring.
+ * The return value is in range from 0 to 100.
+ */
+u32 hv_ringbuf_avail_percent(struct hv_ring_buffer_info *ring_info) {
+	u32 avail_read, avail_write;
+
+	hv_get_ringbuffer_availbytes(ring_info, &avail_read, &avail_write);
+
+	return avail_write * 100 / hv_get_ring_buffersize(ring_info);
+}
+EXPORT_SYMBOL(hv_ringbuf_avail_percent);

That makes no sense, what happens 1 second later to the buffer?  You can't
expect this result to be valid anymore, right?

It's not necessary to be very precise for the flow control, just a rough 
estimate on how full the buffer is enough. We only want to keep a small
amount of buffer to be available, so the outgoing packets don't need to
be retried.

quoted

--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c

@@ -51,6 +51,16 @@ static int ring_size = 128;

module_param(ring_size, int, S_IRUGO);  MODULE_PARM_DESC(ring_size,
"Ring buffer size (# of pages)");

+uint ring_avail_percent_hiwater = 20;
+module_param(ring_avail_percent_hiwater, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(ring_avail_percent_hiwater,
+	"Ring buffer available percentiles to wake up xmit queue");
+
+uint ring_avail_percent_lowater = 10;
+module_param(ring_avail_percent_lowater, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(ring_avail_percent_lowater,
+	"Ring buffer available percentiles to stop xmit queue");

Eeek, no, how in the world is someone supposed to know to set these things?

Please make this work "automatically", otherwise no one will ever get it right.
Don't make it tunable as a way out of making a decision on how the driver
should work.

I will remove the tunables. The default values here work fine in our tests.

Thanks,
- Haiyang