[PATCH 3/3] net: mhi: Add dedicated alloc thread

[Loic Poulain <loic.poulain@linaro.org>]

The buffer allocation for RX path is currently done by a work executed
in the system workqueue. The work to do is quite simple and consists
mostly in allocating and queueing as much as possible buffers to the MHI
RX channel.

It appears that using a dedicated kthread would be more appropriate to
prevent
1. RX allocation latency introduced by the system queue
2. Unbounded work execution, the work only returning when queue is
full, it can possibly monopolise the workqueue thread on slower systems.

This patch replaces the system work with a simple kthread that loops on
buffer allocation and sleeps when queue is full. Moreover it gets rid
of the local rx_queued variable (to track buffer count), and instead,
relies on the new mhi_get_free_desc_count helper.

After pratical testing on a x86_64 machine, this change improves
- Peek throughput (slightly, by few mbps)
- Throughput stability when concurrent loads are running (stress)
- CPU usage, less CPU cycles dedicated to the task

Below is the powertop output for RX allocation task before and after
this change, when performing UDP download at 6Gbps. Mostly to highlight
the improvement in term of CPU usage.

older (system workqueue):
Usage       Events/s    Category       Description
63,2 ms/s     134,0        kWork          mhi_net_rx_refill_work
62,8 ms/s     134,3        kWork          mhi_net_rx_refill_work
60,8 ms/s     141,4        kWork          mhi_net_rx_refill_work

newer (dedicated kthread):
Usage       Events/s    Category       Description
20,7 ms/s     155,6        Process        [PID 3360] [mhi-net-rx]
22,2 ms/s     169,6        Process        [PID 3360] [mhi-net-rx]
22,3 ms/s     150,2        Process        [PID 3360] [mhi-net-rx]

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
---
 drivers/net/mhi_net.c | 98 ++++++++++++++++++++++++++-------------------------
 1 file changed, 50 insertions(+), 48 deletions(-)

diff --git a/drivers/net/mhi_net.c b/drivers/net/mhi_net.c
index 0333e07..eef40f5 100644
--- a/drivers/net/mhi_net.c
+++ b/drivers/net/mhi_net.c
@@ -5,6 +5,7 @@
  */
 
 #include <linux/if_arp.h>
+#include <linux/kthread.h>
 #include <linux/mhi.h>
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
@@ -25,7 +26,6 @@ struct mhi_net_stats {
 	u64_stats_t tx_bytes;
 	u64_stats_t tx_errors;
 	u64_stats_t tx_dropped;
-	atomic_t rx_queued;
 	struct u64_stats_sync tx_syncp;
 	struct u64_stats_sync rx_syncp;
 };
@@ -33,17 +33,59 @@ struct mhi_net_stats {
 struct mhi_net_dev {
 	struct mhi_device *mdev;
 	struct net_device *ndev;
-	struct delayed_work rx_refill;
+	struct task_struct *refill_task;
+	wait_queue_head_t refill_wq;
 	struct mhi_net_stats stats;
 	u32 rx_queue_sz;
 };
 
+static int mhi_net_refill_thread(void *data)
+{
+	struct mhi_net_dev *mhi_netdev = data;
+	struct net_device *ndev = mhi_netdev->ndev;
+	struct mhi_device *mdev = mhi_netdev->mdev;
+	int size = READ_ONCE(ndev->mtu);
+	struct sk_buff *skb;
+	int err;
+
+	while (1) {
+		err = wait_event_interruptible(mhi_netdev->refill_wq,
+					       !mhi_queue_is_full(mdev, DMA_FROM_DEVICE)
+					       || kthread_should_stop());
+		if (err || kthread_should_stop())
+			break;
+
+		skb = netdev_alloc_skb(ndev, size);
+		if (unlikely(!skb)) {
+			/* No memory, retry later */
+			schedule_timeout_interruptible(msecs_to_jiffies(250));
+			continue;
+		}
+
+		err = mhi_queue_skb(mdev, DMA_FROM_DEVICE, skb, size, MHI_EOT);
+		if (unlikely(err)) {
+			net_err_ratelimited("%s: Failed to queue RX buf (%d)\n",
+					    ndev->name, err);
+			kfree_skb(skb);
+			break;
+		}
+
+		/* Do not hog the CPU */
+		cond_resched();
+	}
+
+	return 0;
+}
+
 static int mhi_ndo_open(struct net_device *ndev)
 {
 	struct mhi_net_dev *mhi_netdev = netdev_priv(ndev);
 
-	/* Feed the rx buffer pool */
-	schedule_delayed_work(&mhi_netdev->rx_refill, 0);
+	mhi_netdev->refill_task = kthread_run(mhi_net_refill_thread, mhi_netdev,
+					      "mhi-net-rx");
+	if (IS_ERR(mhi_netdev->refill_task)) {
+		return PTR_ERR(mhi_netdev->refill_task);
+	}
 
 	/* Carrier is established via out-of-band channel (e.g. qmi) */
 	netif_carrier_on(ndev);
@@ -57,9 +99,9 @@ static int mhi_ndo_stop(struct net_device *ndev)
 {
 	struct mhi_net_dev *mhi_netdev = netdev_priv(ndev);
 
+	kthread_stop(mhi_netdev->refill_task);
 	netif_stop_queue(ndev);
 	netif_carrier_off(ndev);
-	cancel_delayed_work_sync(&mhi_netdev->rx_refill);
 
 	return 0;
 }
@@ -138,9 +180,6 @@ static void mhi_net_dl_callback(struct mhi_device *mhi_dev,
 {
 	struct mhi_net_dev *mhi_netdev = dev_get_drvdata(&mhi_dev->dev);
 	struct sk_buff *skb = mhi_res->buf_addr;
-	int remaining;
-
-	remaining = atomic_dec_return(&mhi_netdev->stats.rx_queued);
 
 	if (unlikely(mhi_res->transaction_status)) {
 		dev_kfree_skb_any(skb);
@@ -163,9 +202,8 @@ static void mhi_net_dl_callback(struct mhi_device *mhi_dev,
 		netif_rx(skb);
 	}
 
-	/* Refill if RX buffers queue becomes low */
-	if (remaining <= mhi_netdev->rx_queue_sz / 2)
-		schedule_delayed_work(&mhi_netdev->rx_refill, 0);
+	if (mhi_get_free_desc_count(mhi_dev, DMA_FROM_DEVICE) >= mhi_netdev->rx_queue_sz / 3)
+		wake_up_interruptible(&mhi_netdev->refill_wq);
 }
 
 static void mhi_net_ul_callback(struct mhi_device *mhi_dev,
@@ -200,42 +238,6 @@ static void mhi_net_ul_callback(struct mhi_device *mhi_dev,
 		netif_wake_queue(ndev);
 }
 
-static void mhi_net_rx_refill_work(struct work_struct *work)
-{
-	struct mhi_net_dev *mhi_netdev = container_of(work, struct mhi_net_dev,
-						      rx_refill.work);
-	struct net_device *ndev = mhi_netdev->ndev;
-	struct mhi_device *mdev = mhi_netdev->mdev;
-	int size = READ_ONCE(ndev->mtu);
-	struct sk_buff *skb;
-	int err;
-
-	while (atomic_read(&mhi_netdev->stats.rx_queued) < mhi_netdev->rx_queue_sz) {
-		skb = netdev_alloc_skb(ndev, size);
-		if (unlikely(!skb))
-			break;
-
-		err = mhi_queue_skb(mdev, DMA_FROM_DEVICE, skb, size, MHI_EOT);
-		if (unlikely(err)) {
-			net_err_ratelimited("%s: Failed to queue RX buf (%d)\n",
-					    ndev->name, err);
-			kfree_skb(skb);
-			break;
-		}
-
-		atomic_inc(&mhi_netdev->stats.rx_queued);
-
-		/* Do not hog the CPU if rx buffers are consumed faster than
-		 * queued (unlikely).
-		 */
-		cond_resched();
-	}
-
-	/* If we're still starved of rx buffers, reschedule later */
-	if (unlikely(!atomic_read(&mhi_netdev->stats.rx_queued)))
-		schedule_delayed_work(&mhi_netdev->rx_refill, HZ / 2);
-}
-
 static int mhi_net_probe(struct mhi_device *mhi_dev,
 			 const struct mhi_device_id *id)
 {
@@ -256,7 +258,7 @@ static int mhi_net_probe(struct mhi_device *mhi_dev,
 	mhi_netdev->mdev = mhi_dev;
 	SET_NETDEV_DEV(ndev, &mhi_dev->dev);
 
-	INIT_DELAYED_WORK(&mhi_netdev->rx_refill, mhi_net_rx_refill_work);
+	init_waitqueue_head(&mhi_netdev->refill_wq);
 	u64_stats_init(&mhi_netdev->stats.rx_syncp);
 	u64_stats_init(&mhi_netdev->stats.tx_syncp);
 
-- 
2.7.4