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Subject[PATCH 6/6] Drivers: hv: vmbus: do a fair round robin when selecting an outgoing channel
Date
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@redhat.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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