summaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorK. Y. Srinivasan <kys@microsoft.com>2016-07-01 16:26:37 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-08-31 13:05:41 +0200
commit3724287c0ec472815ebe5ae3790f77965c6aa557 (patch)
tree7a7364937447691888e458e854f0ee688ecce419
parent8de0d7e951826d7592e0ba1da655b175c4aa0923 (diff)
downloadlinux-3724287c0ec472815ebe5ae3790f77965c6aa557.tar.bz2
Drivers: hv: vmbus: Implement a mechanism to tag the channel for low latency
On Hyper-V, performance critical channels use the monitor mechanism to signal the host when the guest posts mesages for the host. This mechanism minimizes the hypervisor intercepts and also makes the host more efficient in that each time the host is woken up, it processes a batch of messages as opposed to just one. The goal here is improve the throughput and this is at the expense of increased latency. Implement a mechanism to let the client driver decide if latency is important. Signed-off-by: K. Y. Srinivasan <kys@microsoft.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--drivers/hv/channel.c7
-rw-r--r--include/linux/hyperv.h35
2 files changed, 41 insertions, 1 deletions
diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
index 9a49505a8e06..884cebf5f9b9 100644
--- a/drivers/hv/channel.c
+++ b/drivers/hv/channel.c
@@ -43,7 +43,12 @@ static void vmbus_setevent(struct vmbus_channel *channel)
{
struct hv_monitor_page *monitorpage;
- if (channel->offermsg.monitor_allocated) {
+ /*
+ * For channels marked as in "low latency" mode
+ * bypass the monitor page mechanism.
+ */
+ if ((channel->offermsg.monitor_allocated) &&
+ (!channel->low_latency)) {
/* Each u32 represents 32 channels */
sync_set_bit(channel->offermsg.child_relid & 31,
(unsigned long *) vmbus_connection.send_int_page +
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 50f493eedeb5..755e8f566a47 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -850,6 +850,31 @@ struct vmbus_channel {
* ring lock to preserve the current behavior.
*/
bool acquire_ring_lock;
+ /*
+ * For performance critical channels (storage, networking
+ * etc,), Hyper-V has a mechanism to enhance the throughput
+ * at the expense of latency:
+ * When the host is to be signaled, we just set a bit in a shared page
+ * and this bit will be inspected by the hypervisor within a certain
+ * window and if the bit is set, the host will be signaled. The window
+ * of time is the monitor latency - currently around 100 usecs. This
+ * mechanism improves throughput by:
+ *
+ * A) Making the host more efficient - each time it wakes up,
+ * potentially it will process morev number of packets. The
+ * monitor latency allows a batch to build up.
+ * B) By deferring the hypercall to signal, we will also minimize
+ * the interrupts.
+ *
+ * Clearly, these optimizations improve throughput at the expense of
+ * latency. Furthermore, since the channel is shared for both
+ * control and data messages, control messages currently suffer
+ * unnecessary latency adversley impacting performance and boot
+ * time. To fix this issue, permit tagging the channel as being
+ * in "low latency" mode. In this mode, we will bypass the monitor
+ * mechanism.
+ */
+ bool low_latency;
};
@@ -891,6 +916,16 @@ static inline void set_channel_pending_send_size(struct vmbus_channel *c,
c->outbound.ring_buffer->pending_send_sz = size;
}
+static inline void set_low_latency_mode(struct vmbus_channel *c)
+{
+ c->low_latency = true;
+}
+
+static inline void clear_low_latency_mode(struct vmbus_channel *c)
+{
+ c->low_latency = false;
+}
+
void vmbus_onmessage(void *context);
int vmbus_request_offers(void);