linux-kernel - [PATCH 6/6] Drivers: hv: vmbus: do a fair round robin when selecting an outgoing channel

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Message-Id: <1429626460-7947-7-git-send-email-vkuznets@redhat.com>
Date:	Tue, 21 Apr 2015 16:27:40 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	Haiyang Zhang <haiyangz@...rosoft.com>,
	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	Dexuan Cui <decui@...rosoft.com>
Subject: [PATCH 6/6] Drivers: hv: vmbus: do a fair round robin when selecting an outgoing channel

vmbus_get_outgoing_channel() implements the following algorithm for selecting
an outgoing channel (despite the comment before the function saying it
distributes the load equally):
1) If we have no subchannels return the primary channel;
2) If primary->next_oc is grater than primary->num_sc reset the primary->next_oc
   to 0 and return the primary channel;
3) Aim for the primary->next_oc subchannel, increment primary->next_oc;
4) Loop through all opened subchannels. If we see a channel which has
 target_cpu == current_cpu return it. If we reached the primary->next_oc'th
open subchannel return it;
5) Return the primary channel.
The implementation also skips the subchannel No. 0 unless it matches the current
cpu as we assign i to 1 in the initialization.

This is not a fair round robin as subchannels in the beginning of the list are
more likely to be returned and checking for current cpu aslo creates additional
complexity. Simplify the vmbus_get_outgoing_channel() function, make it do what
the comment before it says.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 drivers/hv/channel_mgmt.c | 27 +++++++++------------------
 1 file changed, 9 insertions(+), 18 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index daa6417..df82442 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -827,39 +827,30 @@ cleanup:
 struct vmbus_channel *vmbus_get_outgoing_channel(struct vmbus_channel *primary)
 {
 	struct list_head *cur, *tmp;
-	int cur_cpu;
 	struct vmbus_channel *cur_channel;
-	struct vmbus_channel *outgoing_channel = primary;
-	int next_channel;
-	int i = 1;
+	int i = 0;

 	if (list_empty(&primary->sc_list))
-		return outgoing_channel;
+		return primary;

-	next_channel = primary->next_oc++;
-
-	if (next_channel > (primary->num_sc)) {
+	if (primary->next_oc > primary->num_sc) {
 		primary->next_oc = 0;
-		return outgoing_channel;
+		return primary;
 	}

-	cur_cpu = hv_context.vp_index[get_cpu()];
-	put_cpu();
 	list_for_each_safe(cur, tmp, &primary->sc_list) {
+		i++;
 		cur_channel = list_entry(cur, struct vmbus_channel, sc_list);
 		if (cur_channel->state != CHANNEL_OPENED_STATE)
 			continue;

-		if (cur_channel->target_vp == cur_cpu)
-			return cur_channel;
-
-		if (i == next_channel)
+		if (i > primary->next_oc) {
+			primary->next_oc = i;
 			return cur_channel;
-
-		i++;
+		}
 	}

-	return outgoing_channel;
+	return primary;
 }
 EXPORT_SYMBOL_GPL(vmbus_get_outgoing_channel);

-- 
1.9.3

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